Q and answers

Phase Test (attempt March 2008). Q1. Fill in the missing words mark) Protein targeting to the endoplasmic reticulum is generally ___co______.-translational,
whereas targeting to mitochondria is generally _ post__- translational.

Q2. Proteins targeted initially to the endoplasmic reticulum may finally be sorted to:(1 mark)
A) the endoplasmic reticulum membrane
B) the endoplasmic reticulum lumen
C) lysosomes
D) the plasma membrane
E) all of the aboveANSWER = __E_____
Q3. Fill in the missing words: (3 marks)

When a protein that is to be targeted to the endoplasmic reticulum is synthesised, as the
signal sequence emerges from the ribosome is will be bound by a __signal__ recognition
_particle_that will stop _translation_and, more importantly, locate the complex
to the _endoplasmic__ __reticulum___by binding to a _receptor_ on the
surface.
Q4. What is the most common modification of proteins in the endoplasmic reticulum, and what, in particular is used to target proteins to lysosomes? (1 mark)

ER modification: co translational glycosylation
Lysosome targeting: Mannose-6-phospahte
Q5. What are two common features of an endoplasmic reticular (or bacterial) signal sequence? (1 mark)

Feature 1: A sequence of hydrophobic amino acids near the amino terminal
Feature 2: Presence of basic amino acids after the hydrophobic amino acids
Q6. How many target sites are there within mitochondria and what are they called?(2 marks)
Mitochondria has three target sites. They are
1. NADH-CoQ Reductase
2. Co Q- cytochrome c Reductase
3. Cytochrome c oxidase
Q7. Name two main groups of molecular chaperones and say why they so called:
(1 mark)
Group i) Heat shock proteins HSP 47
Group ii)Endoplasmic reticulum protein 29 ERp29
They are so called because heat shock proteins are released when the cells are exposed to stress and ERp29 is known so because the gene ERp29 encodes for it.

Q8. Which molecular chaperone may have a role in the development of biological species?
(1 mark)
Chaperone: BiP(immunoglobulin heavy chain binding protein)
What is its normal role in cells?

This chaperon has a role in the binding of the heavy immunoglobulin chains which are abnormally folded.
Q9. What is the name of the major spongiform encephalophy found in humans?(2 marks)
Human SE: Creutzfeldt – Jakob disease
Name THREE classes of this disease (different ways in which people become victims):
i) It occurs by iatrogenic transmission particularly in transplant of cornea or via growth hormone.
ii) It can occur genetically if a mutation occurs in germ cells.
iii) It can be caused sporadically that is without any cause or via a mutation in somatic cells.
Q10. What is the name for the infective form of prion protein? (3 marks)

Name: Prion protein scrapie
How is this protein thought to bring about the disease in a susceptible host?
Answer: Normally this prion protein exists in the form of alpha pleated sheet but in the infectious form it changes to beta pleated sheet. This beta pleated sheet has the ability to convert other normal forms to infective forms and aggregate into filaments.
Why are prnp0/0 (knockout) mice not susceptible to infection with the infective from of the prion?
Answer: This is because these mice lack the gene which forms prion protein.
Q11. Calculate and filI the missing numbers in the protein purification table for three chromatographic procedures below:(4 marks)
Procedure
Total
protein
(mg)
Total Enzyme
Activity
(nKat)
Specific
activity
Yield (%)
Purification
factor
Ion exchange
355
10780
100
1
Gel filtration
78
8475
Affinity
5
3462
Please show your calculations.
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Q12.
This question (parts a, b c and d) refers to the metabolic pathways and key enzymes illustrated in Figure A.
a. What are the missing metabolite in Box A and Box B? (1 mark)
Box A……Oxaloacetate……………………
Box B……Triacylglycerol…………………
b. Annotate Figure 1 with the following pathways. (2 marks)
Glycolysis
-oxidation of fatty acids
Ensure that the direction as well as the specific metabolites where the pathway starts and stops is clear.
c. Annotate Figure 1 with the following enzymes (2 marks)
Pyruvate kinase
TAG Lipase
Ensure that it is obvious which reaction the enzyme is linked to.
d. Where does GLUCOSE enter the metabolic pathways outlined in Figure A? (1 mark)

ANSWER = _B___
Figure A:
Q13.
a. Complete Figure 2 “ Metabolic decisions at the Glucose-6-Phosphate crossroads of mammalian intermediary metabolism”
The answer required may be a molecule or a pair of pathways. (3 marks)
With reference to Figure 2:
b. This is one of the major switch points in mammalian intermediary metabolism. Which of
the following is not a choice that is made at this crossroads? (1 mark)
A) Energy storage
B) Aerobic or anaerobic state of the cell
C) ATP generation
D) Biosyntheses
E) NADPH generation
F) NADH generationANSWER = B_____
Q14.
What is a futile cycle? Explain how a cell/organism avoids the operation of futile cycles.
(2 marks)
Answer: A futile cycle is one in which the catabolic and anabolic pathways operate at the same time and hence there is a waste of energy. A cell avoids the operation of futile cycles with the assistance of distinctive enzymes which regulate specific pathways and hence prevent the operation of both pathways at the same time. These distinctive enzymes function in different ways.
Q15.
Fill the gaps in the following paragraph (2 marks)
The ketone bodies, acetoacetate_, - _hydroxybutyrate__ and acetone, are overproduced during
fasting, when fatty acids from stored ___lipids__ become the principle oxidizable fuel.
Accumulation of _acetyl__CoA and its precursor acetylacetyl-CoA favours ketone body
formation. Because oxaloacetate is used for _gluconeogenesis__, it is withdrawn from the
__citric acid cycle__, bringing this to a near halt. The acetyl-CoA that is produced by ­­­­­­­­­­­­­­­­­­­­­
__beta oxidation___ of fatty acids can no longer be oxidized via the
_ citric acid cycle _ so it accumulates.
Q16.
Which reactions do the following enzymes catalyse? Give the full, balanced reaction.
(2 marks)
Protein kinaseProtein + ATP ———-protein kinase——–ADP + Protein-PO4
Adenyl cyclase ATP ———Adenyl cyclise—-cAMP
Q17.
Define the following terms with respect to hormone action in mammalian cells. (2 marks)
Hormone
A hormone is a substance which is produced by the specific cells and is carried to the target site where it acts on particular cells and produce particular responses.
Receptor
A receptor is a molecule which has the capability of binding to specific substances like hormones and they integrate their responses.
Q18.
Explain clearly the effect of insulin on a hepatocyte. (2 marks)
Answer: Insulin does not play a role in the uptake of glucose in the liver but it has other effects. It increases the synthesis of fatty acids, glycogen and protein synthesis. On the other hand it decreases gycogenolysis and the formation of ketone bodies by the liver.
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Q19.
What type of food yields the highest amount of energy per gram?(1 mark)
A. Carbohydrate
B. Fat
C. Protein
D. AlcoholANSWER = _B__
Q20.
List the four principal factors that will influence an individual’s energy expenditure (2 marks)
i) Physical acivity
ii) Normal hormonal balance
iii) Diet of a person
iv) Stress
Q21
Active forms of most enzymes that digest food may normally be found in all of the following except(1 mark)
A. In soluble form in the lumen of the stomach
B. In the saliva
C. Attached to the luminal surface of the plasma membrane of intestinal epithelial cells
D. In zymogen granules of pancreatic exocrine cells
ANSWER = __C__
Q22.
Cholesystokinin is a potent secretagogue of (1 mark)
A. Amylase by the salivary glands
B. HCl by the stomach
C. Gastrin by the stomach
D. Hydrolytic enzymes by the pancreasANSWER = _D_
Q23.
Briefly describe how lipids are digested.
(2 marks)
Answer: The digestion of lipids starts in the mouth where it is acted upon by lingual lipase. In the stomach the action of gastric lipase begins. These two lipases are not great contributors in the digestion of lipids and they act mainly on short to medium chain fatty acids which include the milk fats. Small intestine is the main site where lipid digestion takes place. This occurs with the help of pancreatic lipase, phospholipase A2 and cholesterol esterase. The bile salts also play a role in digestion by emulsification which is the breakdown of large fat droplets in to small ones so that the enzymes have a larger surface area to carry out their function.
Q24.
Explain the function of GLUT in the small intestine (1 mark)
Answer: The GLUT in the small intestine is responsible for the absorption of glucose.
Q25.
Briefly describe how the active transport of glucose occurs in the small intestine.
(2 marks)
Answer: Transport of glucose in the small intestine is associated with sodium ions. The sodium ions and glucose share the same transporter and the transport of glucose occurs by cotransport with sodium and is referred to as secondary active transport. The concentration of the sodium in the intestinal cells is low. This causes the movement of sodium in the cells and hence the movement of glucose as it is transported with the sodium ions. The glucose is then moved across the interstitium into the capillaries whereas the sodium ions move into intercellular spaces. The provision of energy for this process is indirect by the active transport of sodium out of the cell. This is the reason why this transport of glucose is referred to as secondary active transport.
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