01. 01. Which of the following would occur in untreated diabetes?
A. A. Hypoglycemia with elevated blood
sugar.
B. B. Ketosis and hypoglycemia.
C. C. Ketosis and elevated blood sugar.
D. D. Hyperglycemia accompanied by
scurvy.
02. 02. A defect in which of the following enzymes is likely to have
the most severe effect on protein digestion?
A. A. Amylase
B. B. Carboxypeptidase A
C. C. Chymotrypsin
D. D. Elastase
E. E. Trypsin
03. 03. Characteristics commonly found in conditions which involve
abnormalities in porphyrin metabolism include all of
the following EXCEPT
A. A. skin sensitivity.
B. B. neurologic disturbances.
C. C. decreased dietary requirement for
protein
D. D. erythrocyte fragility.
E. E. liver disease
04. 04. Which of the following is not required in the diet of a
healthy adult human?
A. A. cysteine
B. B. isoleucine
C. C. lysine
D. D. methionine
E. E. phenylalanine
05. 05. Hyponatraemia is usually
found in all of following EXCEPT:
A. A. Severe liver disease
B. B. Bronchial carcinoma
C. C. Cushings syndrome
D. D. Ingestion of ecstasy
E. E. Untreated congestive cardiac
failure
06. 06. Folic acid
A. A. is absorbed predominantly in the
jejunum
B. B. body stores are adequate for 3
years
C. C. is effective treatment for
alcohol-induced macrocytosis
D. D. bioavailability is improved by
cooking
E. E. blood level is reduced in
stagnant loop syndrome
07. 07. If a child develops cataracts and a lowering of intelligence
after drinking milk regularly he/she is probably suffering from:
A. A. lactose intolerance.
B. B. diabetes.
C. C. galactosemia.
D. D. disaccharidase deficiency.
08. 08. Vitamin A deficiency can result in
A. A. weakness of left-right eye
movement.
B. B. dermatitis, diarrhea, and
dementia.
C. C. night blindness and Bitot's spots.
D. D. eye strain, sensitivity to light
and cracks at corners of mouth.
E. E. increased fragility of red blood
cell membranes.
09. 09. One reason that exercise is beneficial in the treatment of
Type II diabetes mellitus is that
A. A. fatty acid synthesis is activated
in the liver.
B. B. glucose transporters are
activated in the liver.
C. C. glucose transporters are
activated in the muscle.
D. D. ketone body
synthesis is inhibited in the liver.
E. E. glycogen breakdown is inhibited in
the muscle.
10. 10. A deficiency in glucose 6-phosphatase will cause all of the
following EXCEPT:
A. A. Fasting hypoglycemia
B. B. Negative blood glucose response
to galactose injection
C. C. Lactic acidosis
D. D. Hyperlipidemia
E. E. Abnormal glycogen structure
11. 11. G proteins are
A. A. growth factors
B. B. part of cell surface receptors
C. C. Immunoglobulins
D. D. Albumins
E. E. nuclear proteins
12. 12. Human breast milk
A. A. can be safely used in babies with
galactosaemia
B. B. contains more protein than cow's
milk
C. C. contains less fat than cow's milk
D. D. has a relatively high sodium
content
E. E. contains less carbohydrate than
cow's milk
13. 13. Bile acids
A. A. are formed in the pancreas from
cholesterol
B. B. are conjugated with taurine and glycine before
excretion into bile
C. C. are about 50% reabsorbed from the
intestinal lumen
D. D. cannot be metabolised
by intestinal bacteria
E. E. are synthesised
at up to 10mg/day in normal individuals
14. 14. Protamine:
A. A. is a basic
protein
B. B. 1mg antagonises 100 mg heparin
C. C. is a
myocardial stimulant
D. D. is
contraindicated in hepatic failure
E. E. is 60% protein
bound
15. 15. When the level of glucose in the blood becomes very low to
the point a person feels faint or goes into a coma, the condition is called:
A. A. hypotension.
B. B. diabetes.
C. C. starvation.
D. D. hypoglycemia.
E. E. Diabetic ketosis.
16. 16. Following statement concerning nephritic syndrome are true
EXCEPT:
A. A. There is an increase in the
interstitial fluid volume and oedema.
B. B. There is renal retention of
sodium and water.
C. C. There is an increase in glomerular capillary permeability to albumin leading to proteinuria.
D. D. Lipiduria may lead to
the presence of oval fat bodies in the urine.
E.
17. 17. If a person had glossitis, cheilosis and cataracts you might suspect a deficiency of:
A. A. niacin.
B. B. riboflavin.
C. C. vitamin C.
D. D. thiamin.
E. E. Vitamin A
18. 18. Which of the following plays a major role in regulating the
elimination of alcohol from the body?
A. A. Liver alcohol dehydrogenase
B. B. muscle hexokinase
C. C. Brain acetaldehyde dehydrogenase
D. D. Kidney antidiuretic
hormone
E. E. Lung respiratory rate
19. 19. which one of the following statements concerning gluconeogeenesis is false:
A. A. In mammals gluconeogenesis
occurs in liver
B. B. In mammals gluconeogenesis
occurs in adipose tissue
C. C. In mammals gluconeogenesis
occurs in kidney
D. D. The pathway is stimulated by
glucagons
E. E. The pathway is inhibited by
insulin
20. 20. Tyrosine, in addition to being found in protein, can also be
converted to:
A. A. Tryptophan.
B. B. Serotonin (in the brain).
C. C. Adrenalin (in the adrenal gland).
D. D. thyroid hormones.
E. E. ACTH
01. During lyposomal digestion
lipid is oxidized to a chemically heterogeneous pigmented substance called:
A. xanthocyanin
B. lipofuscin
C. melanin
D. lectoferrin
E. none of
above
02. Which of the following lipids contains phosphate?
A. cholesterol
B. cerebroside
C. testosterone
D. diacylglycerol
E. sphingomyelin
F. ceramide
03. The major connective tissue matrix component that is
degraded in patients with emphysema is
A. collagen.
B. Mucopolysaccharide
C. Elastin
D. basement
membrane
E. gamma
globulins
04. Proteins in the blood, which control rates of chemical
reactions, and are called chemical messengers, are known as:
A. catalysts.
B. enzymes.
C. amino acids.
D. hormones.
E. Free fatty
acids
05. Acute intermittent porphyria
is the result of a defect in the biosynthetic pathway for:
A. collagen
B. corticosteroid
C. fatty acid
D. glucose
E. heme
06. Persons suffering from Von Gierke's
disease
A. cannot
convert glucose-6-phosphate to glucose
B. have
enlarged livers
C. cannot
convert glycogen to glucose
D. All of the
above are correct
07. The intracellular triglyceride lipase in adipose tissue
is:
A. activated
by the action of insulin.
B. not
affected by adrenalin.
C. inhibited
when phosphorylated by the cAMP
dependent protein kinase.
D. activated
by the action of glucagon.
E. inhibited
by malonyl CoA.
08. Which of the following antibiotics might allow the
formation of a peptide dimer?
A. Erythromycin
B. Chloramphenicol
C. Tetracycline
D. Streptomycin
09. All of the
following statements regarding albumin are true EXCEPT:
A. has a
molecular weight of 69000 with a strong negative charge on its surface
B. is
synthesized by the hepatocytes at the rate of
9-12gm/day in a healthy adult
C. generates oncotic pressure because of its high mol weight
D. has a
degradation half-life of 18 days
E. can be
present in urine in individuals on prolonged standing
10. In a long-term fast (more than 30 hours) most of the
blood glucose is derived from
A. acetyl-CoA.
B. amino
acids.
C. muscle
glycogen.
D. lactic
acid.
E. liver
glycogen.
11. When cells in the brain shrink:
A. the
production of antidiuretic hormone ceases.
B. the kidneys
start excreting more water.
C. they
accumulate a lot of water.
D. the thirst
signal is turned on.
12. If a person absorbs excessive amounts of iron, the
condition is called:
A. muscle
rigor.
B. anemia.
C. hemochromatosis.
D. the
hemoglobin syndrome.
E. Hemolytic jundice
13. An enzyme which joins DNA molecules together is called
A. a
restriction endonuclease.
B. a DNA lyase
C. a DNA ligase.
D. a DNA
polymerase.
E. a reverse
transcriptase
14. The DNA/histone 'beads' are
called:
A. Polysomes.
B. Mesosomes.
C. Nucleosomes.
D. Lysosomes.
15. Which of the following are NOT directly coded by base
sequences in DNA:
A. structural proteins
B. enzymes
C. transfer RNAs
D. lipid
membranes
E. ribosomal RNAs
16. Which of the following hormonal effects is significant
in muscle but not in liver?
A. Epinephrine
stimulated glycogenolysis
B. Insulin
stimulated glycogen synthesis
C. Insulin
stimulated glucose transport
D. Glucagon stimulated gluconeogensis
E. Glucagon stimulated glycogenolysis
17. In heart cells deprived of oxygen during a myocardial
infarction, the PRIMARY cause of the reduction in ATP synthesis is:
A. inhibition
of hexokinase.
B. inhibition
of phosphofructokinase- 1.
C. reduction
in the rate of the TCA cycle.
D. reduction
in the rate of pyruvate oxidation.
E. reduction
in the rate of mitochondrial electron transport.
18. Which of the following would most likely lead to milky
white serum in the fasting state?
A. An excess
of lipoprotein lipase
B. A
deficiency of Apolipoprotein CII
C. An excess
of LDL receptors
D. A
deficiency of nascent HDL
E. An excess
of lecithin cholesterol acyltransferase (LCAT)
19. The two enzyme catalyzed reactions that utilize cobalamin (from Vitamin B12) as a cofactor are the ones
converting:
A. methylmalonyl-CoA to succinyl
CoA and homocysteine to methionine.
B. aspartate to oxaloacetate
and glutamate to y-aminobutyric acid (GABA).
C. tryptophan to serotonin and tyrosine to
3,4dihydroxyphenylalanine (DOPA).
D. norepinephrine to epinephrine and lysine to trimethyllysine.
E. valine to a-ketoisovaleric
acidand a-ketoisovaleric
acid to isobutyryl COA.
20. The biochemical defect in the lysosomes
of cystinotic individuals is the inability to:
A. hydrolyze peptides containing cysteine.
B. oxidize cysteine to cystine.
C. transport
protons.
D. transport cystine.
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After 10 hours of fasting a pacient has large liver glicogen
deposits with short branches that contain only one glucose unit. The most
likely abnormality is caused by defective: |
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Question 2.
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Which one of the following metabolic changes is specific to starvation ? |
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Question 3.
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A 14-year-old boy with type 1 diabetes melitus
missed two insulin injections. What physiopathological
changes would you expect ? |
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Question 4.
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During a game a football player metabolised to
carbon dioxide and water saturated fatty acids with 14-carbons. What is the appropiate number of ATP moles produced by 1 mole of this
fatty acid ? |
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Question 5.
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Gluconeogenesis from glycerol: |
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Question 6.
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A 30-year-old man strarts bodybuilding one hour
twice a week. If the energy is produced during gligolisis
where glucose is converted to lactate which one of the following statements
is most appropiate ? |
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Question 7.
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Which one of the following blood lipoproteins has the highest colesterol content ? |
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Question 8.
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What type of interactions is not involved in tertiary structure of a protein |
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Question 9.
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A high protein diet is associate with: |
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Question 10.
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Which one from the following is an essential aminoacid
with non-ionisable side chain? |
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Q-1 What is the importance of enzymes in living systems?
A-1 Enzymes are the reaction catalysts of the biological
systems.
They have extraordinary catalytic power, often greater than
that of synthetic catalysts. They have high degree of substrate specificity
& accelerate specific chemical reactions. They function in an aqueous
medium under very mild conditions of temperature& pH. Very few
non-biological catalysts show all these properties.
Q-2 If enzymes are not present in body what will happen?
A-2 The biochemical
reactions taking place in living cells at body temperature at sufficiently
rapid in a regular order. Such reactions would have been extremely slow, had
they not been catalyzed by enzymes, which are present in every living cell and
can also act independently of the cell. (e.g.
hydrolysis of starch by salivary amylase).
Q-3 Were enzyme actions were known in earlier time?
A-3 Yes, Biological catalysis was first recognized & described in the early 1800 in studies
of digestion of meat by secretions of stomach & conversion of starch into
sugar by saliva & various plant extracts. In 1850 Louise Pasteur
concluded that fermentation of sugar into alcohol by yeast is catalyzed by”
FERMENTS “& later named than enzymes, are
inseparable from living yeast cells, a view that prevailed for many years.
Q-4 What was Buchner’s important
discovery regarding enzyme? (Or fermentation?)
A-4 In 1897 Buchner showed yeast
extract (cell free) could ferment sugar to alcohol. This encouraged biochemists
to attempt the isolation of many different enzymes & to examine their
catalytic properties
Q-5 What was the important discovery made by James Sumner?
A-5 In 1926 James Sumner crystallized
UREASE& found it is as a protein and postulated that all enzymes are
proteins. This idea remained controversial for some time.
Q-6 What was contribution of J.B.S.HALDANE regarding enzyme?
A-6 During this period (1930) J.B.S.HALDANE wrote a treatise entitled
enzymes, even though molecular nature of enzymes was not fully yet appreciated.
It contained remarkable suggestions of weak bonding interactions and substrate
catalyzed reaction.
Q-7 What is an enzyme?
A-7 Enzymes are the reaction catalysts of the biological systems. They
are protein in nature (With exception of small group of catalytic RNA
molecules). Molecular weight ranges from 12000 to over million. They are
specific in action e.g. Urease Their catalytic
activity depends upon the integrity of their native protein conformation. (If
enzyme is denatured or dissociated into subunits catalytically activity is
usually lost)
The enzymes carry out transformation of molecules and also
mediate transformation of energy e.g. PHOTOSYNTHESIS
Q-8 What are simple and complex enzymes?
A-8 Some enzymes require no chemical groups other than amino acids
for activity, Enzymes composed of only protein are known as simple enzymes.
Complex enzyme composed of protein plus a relatively small organic molecule,
(which is required for enzyme activity)
Q-9 What is apo-enzyme?
A-9 Apo-enzyme is the
protein part of an enzyme without any cofactors or prosthetic group that may be
required for the enzyme to be functional. The apo-enzyme
is catalytically inactive.
Q-10 What is coenzyme or prosthetic group?
A-10 A non-protein
component of an enzyme, which is required for catalytic activity, is known as
co-enzyme or prosthetic group.
Q-11
What is
co-factor?
A-11 Is small organic or inorganic molecules that an apo-enzyme requires for its activity. E.g. Lysine oxidase copper is loosely bound
Q-12 What is the difference between prosthetic group and
co-enzyme?
A-12. A coenzyme or
metal ion that is COVALENTLY bound to the enzyme protein is called prosthetic
group. For e.g. in the cytochromes, the heme prosthetic group is very tightly bound and requires
strong acids to dissociate from its apo enzyme.
Q-13 Do some Enzymes contain vitamin derivatives?
A-12 YES, Many prosthetic groups and coenzymes are water-soluble derivatives
of vitamins. It should be noted that the main clinical symptoms of dietary
vitamin insufficiency generally arise from the malfunction of enzymes, which
lack sufficient cofactors derived from vitamins to maintain homeostasis.
Q-13 What is a holo enzyme?
A-13 A complete catalytically active enzyme together with its
coenzyme and/or metal ions called a holoenzyme.
Apo-enzyme
+ co enzyme ==holoenzyme
Q-14 What is metalloenzyme and metal
activated enzymes?
A-14 Enzymes require a metal in their composition (such as Fe+2,Mg2+. Mn2+,Zn2+ ) are known as metalloenzymes
if they bind and retain their metal atom(s) under all conditions, that is with
very high affinity. Those, which have a lower affinity for metal ion, but still
require the metal ion for activity, are known as metal-activated enzymes.
Nomenclature
Q-15 How enzymes are named?
A-15 Many enzymes have been named by adding the suffix “-ase”
to the name of their substrate or to a word or phrase describing their
activity. e.g. UREASE catalyze hydrolysis of urea,
MALTASE act on maltose, and DNA polymerase catalyze the synthesis of DNA. Other enzymes such as PEPSIN and TRYPSIN have
names they do not denote their substrates.
Q-16 What is IUB
system?
A-16 Sometimes the enzyme may have two or more names, or two different
enzymes have the same name. Because of such ambiguities, and ever increasing
number of newly discovered enzymes a
system for naming and classifying enzymes has been adopted by inter national
agreement, International Union of Biochemistry (IUB-system)
Q-17 How enzymes are classified according to IUB system?
A-17 According to IUB system enzymes are grouped in six major classes.
Each with sub classes based on the type of reaction
catalyzed. Systemic classification of enzymes based on numbering system is
used. Each enzyme is assigned a code number or EC (enzyme commission number)
four-digit classification number and a systematic name, which identifies the
reaction catalyzed.
Q-18 According to IUB system which enzymes are included in group 1?
A-18 OXIDO-REDUCTASE (EC-1)
Enzymes of this group add or
remove hydrogen atoms during the catalysis. They include dehyrogenases
& oxidases, and are mostly concerned with
biological oxidation.
Dehydrogenases removes H+
from substrate in the presence of H+ acceptor group. e.g.
Lactate Dehydrogenase. Enzymatic action can be
represented by:
AH2 +B à A + BH2
OXIDASES – transfer two electrons from the donor to oxygen
resulting usually in hydrogen peroxide formation (H2O2)
e.g. Glucose oxidase. CYTOCHROME oxidase
produces H2O rather than H2O2. Oxygenases catalyze the incorporation of oxygen into a
substrate.
Q-19 According to IUB system which enzymes are included in group 2?
A-19
TRANSFERASES (EC-2)
These enzymes transfer functional groups between donors and
acceptors.
The AMINO, ACYL, PHOSPHATE,
ONE CARBON and GLYCOSYL are the major groups that are transferred. E.g.
A— X+ B à A+ B—X
A. METHYL group---àe.g. Transmethylase
B. ALDEHYDE or KETONIC group e.g. Transaldolase
or transketolase.
C. ACYL GROUP e.g.Aceyltransferase
D. SUGAR GROUP e.g. Glucosyltransferase
E. AMINO-KETO GROUP- Aminotransferase
or transaminases
F. KINASES are specialized trnsferases
that regulate metabolism by transferring phosphate from ATP to other moleculs e.g. Hexokinase:
ATP +Glucose -----àG-6-P+ ADP
Q-20 According to IUB system which enzymes are included in group 3?
A-20 HYDROLASES (EC-3)
A special class of transferases
in which the donor group is transferred to water. The generalized reaction involves the hydrolytic cleavage
of C-O, C-N, O-P and C-S bonds. In other words
enzymes, which add, water to the substrate and hydrolyze or decompose it to
give products.
A—B +
H2O à AH+ BOH
1. LIPASES-------e.g. Glycerol ester hydrolase
2.
PHOSPHATASES------e.g.Glucose-6-Phosphatase
3. CHOLINE ESTERASE hydrolysases acetylcholine
4. PEPTIDASES-----hydrolyses
peptides
5. NUCLEASES e.g.nucleotidase, nucleosidase
6. CARBOHYDRASES e.g. Amylase act on amylose
Lactase, Maltase
7. Enzymes acting on C—N linkage Urease converts
urea into ammonia, Asparginase. Glutaminase,
Arginase
Q-21 According to IUB system which enzymes are included in group 4?
A-21 LYASES (EC4)
Lyases
add or remove water, ammonia, or carbon dioxide from the substrates.
A-B + X-Y ---- > A-X + B-Y
1. DECARBOXYLASE removes CO2 from a orb keto acids or aminoacids.
2. Carbonic
anhydrase
3. Cysteine desulfurase
Q-22 According to IUB system which enzymes are included in group5?
A-22 ISOMERASES (EC5)
A hetrogeneous group of enzymes
catalyze transfer of groups within molecules to yield isomeric forms e.g. isomerazition of! 1)
Optical isomers 2) geometrical isomers
A-----à A’
1. Epimerases or Racemases catalyze inversion at asymmetric carbon atoms
2. MUTASES involve intramolecular
transfer of a group such as a phosporyl
3. CIS-TRANS ISOMERASE e.g. all trans
retinene isomerase
Q-23 According to IUB system which enzymes are included in group 6?
A-23 LIGASES (EC6)
TO liagate
means to bind, Formation of C-C, C-S, C-O& C-N bonds by condensation
reactions. These enzymes carry
out synthetic reactions where two molecules joined at the utilization of a
“high energy phosphate bond of ATP. ”
REACTION: A+B---à A-B
1. Pyruvate carboxylase Pyruvate+CO2+ATP-----àOxaloacetate+ ADP+Pi
2. GLUTAMINE SYNTHETASE
3. Acetyl Co A CARBOXYLASE
Q-24 The following enzyme is classified as EC –2.7.1.1. What does this mean?
A-24 The name of the enzyme catalyzing following reaction is ATP:glucose phospo transferase. Its enzyme classification number (E.C.number) is 2.7.1.1
ATP+ D-Glucose---àD-Glucose-6-phosphate+ ADP
The first digit (2) denotes the class number (transferase)
The second digit (7) denotes sub-class - phospo transferase;
The third digit (1)-Hydroxyl group as an acceptor;
The fourth digit (1) D-glucose as the
phosphate group acceptor.
When the systematic name of an enzyme is long or
cumbersome, a trivial name may be used in this case Hexokinase.
How
glutamete is a prominent intermediate in nitrogen
elimination?
A21.
Q22. Which amino acid related
reaction is therapeutically significant?
A22.
Q23. How clinically important are sterospecific amino acid oxidases?
A23.
Q24. Discribe the role
and significance of glutamate dehydrogenase?
A24.
The reaction catalyzed by glutamate
dehydrogenase is:
NH4+ + α -ketoglutarate
+ NAD (P) H + H+ <----> glutamate + NAD (P)+
+ H2O
Glutamate
dehydrogenase can utilize either NAD orNADP as cofactor.
Q25. Describe the role and
significance of glutamine synthase.
A25.
The reaction catalyzed by glutamine
synthase is:
glutamate + NH4+ +
ATP -------> glutamine + ADP + Pi + H+
The glutamine
synthatase reaction is also important in several
respects. First it produces glutamine, one of the 20 major amino acids. Second,
in animals, glutamine is the major amino acid found in the circulatory system.
Its
role there is to carry ammonia to and from various tissues but principally from
peripheral tissues to the kidney, where the amide nitrogen is hydrolyzed by the
enzyme glutaminase (reaction below); this
process regenerates glutamate and free ammonium ion, which is excreted in the
urine.
glutamine + H2O ------->
glutamate + NH3
Note
that, in this function, ammonia arising in peripheral tissue is carried in a nonionizable form, which has none of the neurotoxic or alkalosis-generating properties of free
ammonia.
Q26. What is Urea Cycle?
A26. About 80% of the excreted
nitrogen is in the form of urea, which is also largely made in the liver, in a
series of reactions that are distributed between the mitochondrial matrix and
the cytosol. The series of reactions that form urea
is known as the Urea Cycle or the Krebs-Henseleit
Cycle.
Q27. What are essential features of
the urea cycle?
A27. The essential features of the
urea cycle reactions and their metabolic regulation are as follows:
1.
1.
Arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase,
generating urea and ornithine.
2.
2. Ornithine arising in
the cytosol is transported to the mitochondrial
matrix, where ornithine transcabamoylase
catalyzes the condensation of ornithine with carbamoyl phosphate, producing citrulline.
The energy for the reaction is provided by the high-energy anhydride of carbamoyl phosphate.
3.
3.
The product, citrulline, is then
transported to the cytosol, where the remaining
reactions of the cycle take place.
4.
4. In a 2-step reaction, catalyzed by cytosolic
argininosuccinate synthetase,
citrulline is converted to argininosuccinate. The
reaction involves the addition of AMP (from ATP) to the amido
carbonyl of citrulline, forming an activated
intermediate on the enzyme surface (AMP-citrulline),
and the subsequent addition of aspartate to form argininosuccinate.
5. 5. Arginine and fumarate are produced from argininosuccinate by the cytosolic
enzyme argininosuccinate lyase. In the final step of the cycle arginase cleaves urea from aspartate,
regenerating cytosolic ornithine,
which can be transported to the mitochondrial matrix for another round of urea
synthesis.
Beginning
and ending with ornithine, the reactions of the cycle
consumes 3 equivalents of ATP and a total of 4 high-energy nucleotide
phosphates. Urea is the only new compound generated by the cycle; all other
intermediates and reactants are recycled.
Q28. How the regulation of the Urea
Cycle takes place in the body?
A28.
The
urea cycle operates only to eliminate excess nitrogen.
On high-protein
diets the carbon skeletons of the amino acids are oxidized for energy or stored
as fat and glycogen, but the amino nitrogen must be excreted.
To
facilitate this process, enzymes of the urea cycle are controlled at the gene
level.
When
dietary proteins increase significantly, enzyme concentrations rise. On return
to a balanced diet, enzyme levels decline. Under conditions of starvation,
enzyme levels rise as proteins are degraded and amino acid carbon skeletons are
used to provide energy, thus increasing the quantity of nitrogen that must be
excreted.
Q29. What happens when excretion of ammonia is deranged?
A29. Built up of ammonia is neurotoxic. Marked brain damage is seen in cases of failure to make
urea via the urea cycle or to eliminate urea through the kidneys. The result of
either of these events is a buildup of circulating levels of ammonium ion.
Aside from its effect on blood pH, ammonia readily traverses the brain blood
barrier and in the brain is converted to glutamate via glutamate dehydrogenase, depleting the brain of α-ketoglutarate. As the α-ketoglutarate
is depleted oxaloacetate falls correspondingly, and
ultimately TCA cycle activity comes to a halt. In the absence of aerobic
oxidative phosphorylation and TCA cycle activity, irreparable
cell damage and neural cell death ensue.
What are vitamins?
A01.
Q02. How chemical
structures of water-soluble vitamins are related to their functions?
Q02. In general, the water soluble vitamins consists of:
o o
Derivatives or
substituted derivatives of sugars (vit-C),
o o
Derivatives of
pyridine (niacin, B6),
o o
Derivatives of purines and pyrimidines (folic
acid, B2, B1),
o o
Amino
acid-organic acid complex (folic acid, biotin, pantothenic
acid) and
o o
A porphyrin-nucleotide complex (B12).
o o As redox agent
on enzyme reactions (Vit-C, B2, B12, folic acid,
niacin)
Q03. Describe briefly
the structure of thiamine and its active form.
A03.
·
· Thiamine is also known as vitamin B1.
·
· Thiamin is derived from a substituted pyrimidine
and a thiazole, which are coupled by a methylene bridge.
·
· Thiamin is rapidly converted to its active form, thiamin
pyrophosphate, TPP, in the brain and liver by a specific enzyme, thiamin diphosphotransferase.
·
· TPP is necessary as a cofactor for the pyruvate
and a-ketoglutarate dehydrogenase
catalyzed reactions as well as the transketolase
catalyzed reactions of the pentose phosphate pathway.
Q04. What is dietary requirement
of Thiamine (Vitamin B1)?
A04. The dietary requirement for thiamine is proportional to the
caloric intake of the diet and ranges from 1.0 - 1.5 mg/day for normal adults.
If the carbohydrate content of the diet is excessive then an increased thiamine
intake will be required. Requirement is increased in pregnancy and lactation.
It also depends of intestinal synthesis and absorption and fat content of diet
(increased Pyruvate).
Q05. What are dietary
sources of Vitamin B1?
A05. Following are the dietary sources of Vitamin B1:
High:
1000-10,000microgram/100g
Wheat germ, rice bran, soybean flour yeast and ham.
Medium:
100-1000microgram/100g
Peanuts,
pecan, walnut, almonds etc. sprouts,
Broccoli,
cauliflower, potatoes, beans,
Eggs, milk and beef whole grain cereals and breads.
Low: 10-100microgram/100g
Apples,
berries, banana, oranges, dates
Beet,
cabbage, carrot, radish, spinach etc.
Q06. Describe biochemical role of
thiamine.
A06.
Q07. Describe clinical
manifestations of thiamine deficiency.
Q08. Describe
briefly the structure of Riboflavin (Vitamin B-2) and its biochemical role.
A08.
Q09. What is dietary
requirement of Riboflavin (Vitamin B-2) and what are dietary sources of it?
A09. The normal daily requirement for riboflavin is 1.2 - 1.7 mg/day
for normal adults.
Following are the dietary sources of Vitamin B-2:
High:
1000-10,000microgram/100g
Beef, chicken, pork, yeast.
Medium:
100-1000microgram/100g
Avocados, currents, asparagus, beans, sprouts, egg, milk,
nuts.
Low:
10-100microgram/100g
Apples,
banana, oranges, dates, carrot, rice
Q10. Describe clinical
manifestations of riboflavin deficiency.
A10. Symptoms associated with riboflavin deficiency include:
inflammation or open sores at the corners of the mouth or lips, a purple -red
inflamed tongue, angular stomatitis, glossitis,
cheilosis, photophobia & seborrheic dermatitis
(dandruff).
Riboflavin
decomposes when exposed to visible light. This characteristic can lead to
riboflavin deficiencies in newborns treated for hyperbilirubinemia
by phototherapy.
Riboflavin
deficiency is often seen in chronic alcoholics due to their poor dietetic
habits.
Q11. Describe
briefly the structure of Niacin and its biochemical role.
A11.
Q12. What
is dietary requirement of Niacin and what are dietary sources of it?
A12. The recommended daily requirement for niacin is 13 - 19 niacin
equivalents (NE) per day for a normal adult. One NE is equivalent to 1 mg of
free niacin).
Following
are the dietary sources of Vitamin B-2:
High: 10-100mg/100g
Peanut, rice bran, liver, heart, Beef, chicken, tuna,
yeast.
Medium: 1-10mg/100g
Avocados, dates, figs, beans, sprouts, nuts.
Low:
0.1-1.0mg/100g
Apples, banana, berries, melon, peach, oranges, sprouts,
tomato
Q13. Describe clinical
manifestations of Niacin deficiency.
A13.
·
· A diet deficient in niacin (as well
as tryptophan) leads to glossitis
of the tongue, dermatitis, weight loss, diarrhea, depression and dementia.
·
· Deficiency in niacin causes pellagra (rough skin).
Pellagra involves the skin and digestive and nervous system. Symptoms are the 4
D's: Dermatitis, Diarrhea, Dementia, & Death. Niacin also has vasodilating activity.
·
·
Several physiological conditions (e.g. Hartnup disease and malignant carcinoid
syndrome) can lead to niacin deficiency.
·
·
In
Hartnup disease tryptophan absorption
is impaired and in malignant carcinoid syndrome tryptophan metabolism is altered resulting in excess
serotonin synthesis.
·
·
Certain drug therapies (e.g. isoniazid) can lead to niacin deficiency. Isoniazid (the hydrazide
derivative of isonicotinic acid) is the primary drug
for chemotherapy of tuberculosis.
·
·
Nicotinic
acid (but not nicotinamide) when administered in
pharmacological doses of 2 - 4 g/day lowers plasma cholesterol levels and has
been shown to be a useful therapeutic for hypercholesterolemia. The major
action of nicotinic acid in this capacity is a reduction in fatty acid
mobilization from adipose tissue. Although nicotinic acid therapy lowers blood
cholesterol it also causes a depletion of glycogen stores and fat reserves in
skeletal and cardiac muscle. Additionally, there is an elevation in blood
glucose and uric acid production. For these reasons nicotinic acid therapy is
not recommended for diabetics or persons who suffer from gout.
Q14. Describe
briefly the structure of Vitamin B-6 and its biochemical role.
A14.
·
· Vitamin B6 is a component of a coenzyme.
Q15. What
is dietary requirement of Vitamin B-6 and what are dietary sources of it?
A15. The requirement for vitamin B6
in the diet is proportional to the level of protein consumption ranging from
1.4 - 2.0 mg/day for a normal adult.
Following
are the dietary sources of Vitamin B-6:
High: 1000-10,000mcg/100g
Walnut,
peanut, wheat germ, brown rice, yeast, liver (Beef), herring, and
Salmon.
Medium: 100-1000mcg/100g
Banana,
Avocados, grapes, pears. Cabbage, carrots, peas, potatoes, tomatoes, spinach, soybean,
wheat, butter and eggs.
Low:
10-100mcg/100g
Apples,
oranges, raisins, watermelon, asparagus, bens, lettuce, onion, cheese and milk
Q16. Describe clinical
manifestations of Vitamin B-6 deficiency.
A16. Deficiency
of Vitamin B6 can cause convulsions, lethargy, mental changes &
retardation, anemia, and skin inflammation.
Deficiencies
of vitamin B6 are rare and usually are related to an overall
deficiency of all the B-complex vitamins.
Isoniazid (see niacin deficiencies above) and penicillamine
(used to treat rheumatoid arthritis and cystinurias)
are two drugs that complex with pyridoxal and pyridoxal phosphate resulting in a deficiency in this
vitamin.
Q17. Write a short note on Pantohenic acid.
A17.
·
·
Deficiency
of pantothenic acid is extremely rare due to its
widespread distribution in whole grain cereals, legumes and meat.
Q18. Write a short
note on Biotin.
A18. Biotin is the prosthetic group
for number of caboxylation reactions e.g.
·
· Pyruvate carboxylase (for synthesis of oxaloacetate for gluconeogensis
and replenishment of citric acid cycle.
·
·
Acetyl-CoA carboxylase (fatty acid
biosynthesis) and
·
·
Propionyl-CoA carboxlase
(methionine, leucine and valine metabolism)
Biotin is found in numerous foods
and also is synthesized by intestinal bacteria and as such deficiencies of the
vitamin are rare.
Deficiencies are generally seen
only after long antibiotic therapies, which deplete the intestinal fauna or
following excessive consumption of raw eggs. The latter is due to the affinity
of the egg white protein, avidin, for biotin
preventing intestinal absorption of the biotin.
Q19. Describe
briefly the structure of Vitamin B-12 and its biochemical role.
A19.
·
·
Vitamin B12
is composed of a complex tetrapyrrol ring structure (corrin ring) and a cobalt ion in the center. It is also
known as cobalamin.
·
·
The vitamin must be hydrolyzed from protein
in order to be active. Hydrolysis occurs in the stomach by gastric acids or the
intestines by trypsin digestion following consumption
of animal meat.
There
are only two clinically significant reactions in the body that require vitamin
B12 as a cofactor.
Q20. Describe source, requirement
and deficiency manifestations of Vitamin B-12.
A20. Vitamin
B12 is not found in plant foods. The main source of B12 in human diet is
through animal products like milk, eggs and liver. Vitamin B12 requires the
presence of intrinsic factor from the stomach in order to be absorbed in the
small intestines. The liver can store up to six years worth of vitamin B-12,
hence deficiencies in this vitamin are rare.
B12 is needed for the efficient production of
blood cells and for the health of the nervous system.
The inability to absorb Vitamin
B12 occurs in pernicious anemia. In pernicious anemia intrinsic factor
is missing. The
anemia results from impaired DNA synthesis due to a block in purine and thymidine
biosynthesis. The block in nucleotide biosynthesis is a consequence of the
effect of vitamin B12 on folate
metabolism. When vitamin B-12 is deficient essentially all of the folate becomes trapped as the N5-methyltetrahydrofolate
derivative as a result of the loss of functional methionine
synthase. This trapping prevents the synthesis of
other tetrahydrofolate derivatives required for the purine and thymidine nucleotide
biosynthesis pathways.
Neurological
complications also are associated with vitamin B-12 deficiency and result from
a progressive demyelination of nerve cells. The demyelination is thought to result from the increase in methylmalonyl-CoA that result from
vitamin B-12 deficiency. Methylmalonyl-CoA is a
competitive inhibitor of malonyl-CoA in fatty acid
biosynthesis as well as being able to substitute for malonyl-CoA
in any fatty acid biosynthesis that may occur. Since the myelin sheath is in
continual flux the methylmalonyl-CoA-induced
inhibition of fatty acid synthesis results in the eventual destruction of the
sheath. The incorporation methylmalonyl-CoA into
fatty acid biosynthesis results in branched-chain fatty acids being produced
that may severely alter the architecture of the normal membrane structure of
nerve cells
Q21. Write
short not on Folic acid and its biochemical role.
A21. The active form of folic acid is folacin.
The
function of THF derivatives is to carry and transfer various forms of
one-carbon units during biosynthetic reactions.
The
one-carbon units are methyl, methylene, methenyl, formyl or formimino groups. These one-carbon transfer reactions are
required in the biosynthesis of serine, methionine, glycine, choline and the purine nucleotides and dTMP.
The
ability to acquire choline and amino acids from the
diet and to salvage the purine nucleotides makes the
role of N5, N10-methylene-THF in dTMP
synthesis the most metabolically significant function for this vitamin.
The
role of vitamin B12 and N5-methyl-THF in the conversion
of homocysteine to methionine
also can have a significant impact on the ability of cells to regenerate needed
THF.
Q22. Describe source, requirement
and deficiency manifestations of Folic acid.
A22.
·
· Folic acid is obtained primarily
from yeasts and leafy vegetables as well as animal liver. Animal cannot
synthesize PABA nor attach glutamate residues to pteroic
acid, thus, requiring folate intake in the diet.
·
· The body needs folic acid and folates
(form of folic acid that occurs in food) to make DNA. Rapidly dividing cells in
the blood, the lining of the colon and developing neutral tube need folic acid
the most.
·
· Folic acid can prevent at least some children from being
born with spina bifida or other birth defects.
·
· Folic acid might prevent heart disease in adults by lowering
levels of an artery damaging substance called homocysteine.
Homocysteine is an amino acid that's used to make
protein. It could damage arteries. Folic acid along with Vitamin B12 &B6
all are needed to convert homocysteine to other
things.
·
· Folic acid is also used in the treatment of sprue- a chronic form of malabsorption.
Q23. Write a short note on Vitamin
C.
A23. Vitamin C is
also known as Ascorbic acid.
·
·
Vitamin
C also is necessary for bone remodeling due to the presence of collagen in the
organic matrix of bones.
Vitamin C is found in fresh fruits
and vegetables including citrus fruits. Vitamin C is necessary for the health
of the supporting tissues of the body such as bone, cartilage and connective
tissue.
Deficiency in vitamin C leads to the disease scurvy due to the
role of the vitamin in the post-translational modification of collagens. Scurvy
is characterized by easily bruised skin, muscle fatigue, soft swollen gums,
decreased wound healing and hemorrhaging, osteoporosis, and anemia.
Vitamin C is readily absorbed and so the primary cause of vitamin
C deficiency is poor diet and/or an increased requirement. The primary
physiological state leading to an increased requirement for vitamin C is severe
stress (or trauma). This is due to a rapid depletion in the adrenal stores of
the vitamin. The reason for the decrease in adrenal vitamin C levels is unclear
but may be due either to redistribution of the vitamin to areas that need it or
an overall increased utilization.