2. Sep 2013
Q1: Enumerate the functions of the following and mention the manifestations of
deficiency of each (5 marks)
a) zinc (2 marks)
b) niacin (3 marks)
A1:
a) zinc is important for :
1- normal growth , reproduction and wound healing
2- storage and release of insulin
3- the function of some enzymes e.g. alkaline phosphatase , carbonic anhydrase
thymidine kinase , carboxypeptidase and retinine reductase.
Zinc deficiency in man leads to impaird taste sensation , poor appetite , retarded growth
, hypogonadism and impaird wound healing.
b) function: nicotinate is the form of niacin required for synthesis of NAD+ (nicotinamide
adenine dinucleotide) and NADP+ (nicotinamide adenine dinucleotide phosphate)
NAD+ and NADP+ are coenzymes of many oxidoreductase enzymes occuring both in
cytosol and within mitochondria.
Generally, NAD+ - linked dehydrogenase catalyze oxidoreduction reactions in oxiditive
pathways e.g. citric acid cycle.
Whereas NADP+ - linked dehydrogenase or reductase are often found in pathways
concerned with reductive biosynthesis e.g. pentose phosphate pathway.
Deficiency lead to pellegra which is characterized by 3 Ds: dermatitis , diarrhea and
dementia (lack of concentration)
Dermatitis is usually seen in skin areas exposed to sunlight and is symmetric. It is in the
form of pigmentation and thickening of the skin. The neurologic symptoms start by
nervous disorders and mental disturbances. In the latter stages dementia occurs.
3. Q2: tabulate the comparison between competitive and non-competitive enzyme inhibtors
(regarding reversibility , effect of substrate concentration and effect on Km and Vmax (2
marks)
A2:
Competitive Non-competitive
Reversibility Reversible (it combines
reversibly with the enzyme
forming an enzyme-inhibtor
complex rather than an
enzyme - substrate complex
(there is no E-I-S complex in
this type)
Reversible (it can combine
with the free enzyme or
with enzyme substrate
complex forming E-I or
ES-I complex
Effect of substrate
concentration
Inhibtion is removed by
increasing the substrate
concentration
No effect of substrate
concentration.
Effect on Km and Vmax In this type of inhibtion the
Vmax of the enzyme is not
decreased , but the apparent
Km is increased i.e the affinity
of the enzyme to the substrate
is decreased.
The I lowers the Vmax of
enzyme but does not
affect the Km (as the
catalytic site is free for
combination with the
substrate)
Q3: mention the rate limiting steps (key enzymes) for the following pathways and
mention the way of regulation of each . (4.5 marks)
a) glycogen breakdown
b) fatty acid synthesis
c) urea formation
A3: a) key enzyme is glycogen phosphorylase
Regulation: 1- covalent modification by hormones: norepinephrine and epinephrine (in
liver and muscle) or glucagon ( in liver only)
2- allosteric regulation: - G-6-P and ATP inhibit glycogen phosphorylase because their
elevated levels indicate that the cell isn't in need for more energy and there is no need
to breakdown glycogen
4. - AMP stimulate glycogen phosphorylase in muscle
-Ca activate phosphorylase b- kinase in muscle and in liver.
b) key enzyme is acetyl coA carboxylase.
Regulation: 1- covalent modification :
- Insulin a) activate acetyl coA carboxylase by dephosphorylation.
b) inhibit the activity of carnitine acyl transferase I, thus blocking the delivery of fatty
acid to B- oxidation.
- Glucagon and catecholamine inhibit acetyl coA carboxylase by phosphorylation.
2-allosteric regulation:
- Acyl coA (palmitoyl coA):
a) is an allosteric inhibtor of acetyl coA carboxylase.
b) it inhibt the transport of citrate from mitochondria to cytosol.
c) inhibt G-6-P dehydrogenase which generate NADPH
-Citrate is an allosteric activator of the enzyme
Long term regulation: insulin induce the synthesis of acetyl coA carboxylase.
c) key regulatory enzymes is carbamoylphosphate synthase.
Regulation: 1- carbamoylphosphate synthase I is the rate limiting enzyme in urea cycle .
It is active only in precence of N-acetylglutamate.(allosteric activator)
2- Excess ammonia formation stimulate urea formation
3- high arginine level stimulate N-acetyl glutamate synthase enzyme thus increase urea
formation
4- high urea level inhibits carbamoylphosphate synthase , ornithine transcarbamoylase
and arginase
5. Q4: write down (using enzymes and coenzymes) biochemical reaction for the fate of:
a) UDP-glucose (3 marks)
b) HMG-CoA (3.5 marks)
c) ceramide (3 marks)
d) homocysteine (3 marks)
e) tyrosine (4 marks)
A4) a)
8. e)
Q5: give an example for each of the following enzymatic reaction : (3 marks)
1- oxidoreductase
2- transferase
3- hydrolase
A5)1- oxidase e.g. oxidase forming water e.g. cytochrome oxidase
2- transglycosylase e.g. UDP-galactose + glucose ~> lactose + UDP
3- esterase e.g. triglyceride ~> glycerol + 3 fatty acid
Q6: mention the enzymatic defects causing the following diseases. (1.5 marks)
a)phenylketonuria
b)hyperammonemia type I
A6) a) phenylalanine hydrolase
b)
9. Q7: state the biochemical names of the compunds A and B and that labeled from 1 to 6
: (3.5 marks)
A7:
A) ATP
B) ADP
1- hexokinase or glucokinase
2- glucose-6-phosphate
3- phosphohexose isomerase
4- phosphofructokinase
5- aldolase
6- glyceraldehyde 3-phosphate
10. Q8: define the metabolic rate and enumerate 4 factors affecting it, indicating which
increase or decrease it. (4 marks)
A8:-
It is the amount of energy liberated per unit of time. It is divided into several components
including performing essential metabolic functions, physical activities , processing of
ingested food and maintenance of body temperature.
1- musclar exercise (increase)
2- sleep ( decrease)
3-stimulation of symphathetic nervous system ( increase)
4- pregnancy and lactation (increase)
5- enviromental temperature ( low temperature increase)
6- fever (increase)
Q9) list 6 changes that occur in response to prolonges starvation (3 marks)
A9) -depletion of food stores in body tissue
-Gradual weight loss
- vitamin deficiency
- Decrease activity of both endocrine and exocrine gland
- Decrease basal metwbolic rate and body temperature
- Decrease cardiovascular function
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