Session 3

Question 0

Discuss the allosteric properties of a key regulatory enzyme

Answer 0

Phosphofructokinase sets pace of glycolysis
Allosterically inhibited by ATP, H+, citrate (T state)
Allosterically activated by AMP, fructose-2,6-bisphosphonate (R state)

Question 1

List the major regulatory mechanisms that control enzyme activity (plus examples)

Answer 1

Short term regulation:
Substrate and product concentration
Change in enzyme conformation - allosteric regulation (activators/inhibitors), covalent modification (phosphorylation), proteolytic cleavage (zymogens, clotting cascade)
Long term regulation:
Change in rate of protein synthesis (enzyme induction/repression)
Change in rate of protein degradation (ubiquitin, proteasome pathway)

Question 2

Discuss the concept of enzyme cascades and the use of protein kinases and phosphatases to regulate activity

Answer 2

Enzyme cascades - allows amplification of the initial signal by several orders of magnitude
Protein kinases - transfer the terminal phosphate
Protein phosphatases - catalyses the hydrolytic removal of phosphoryl groups from proteins

Question 3

Define the term zymogen (plus examples)

Answer 3

Inactive precursors e.g. digestive enzymes

Trypsinogen –> trypsin

Question 4

Explain how activation of the clotting cascade leads to the formation of fibrin

Answer 4

Intrinsic pathway - damage to endothelial lining of blood cells promotes binding of factor XII
Extrinsic pathway - trauma releases tissue factor III
Factor X activation –> thrombin activation –> formation of fibrin clot

Question 5

Discuss the mechanisms that are involved in the regulation of clot formation and breakdown

Answer 5

Factors (proteases), cofactors
Clotting cascade is a highly regulated process
Stopping the process:
1. Dilution of clotting factors by blood and removal by liver
2. Digestion by proteases e.g. protein C
3. Specific inhibitors (AT3) enhanced by heparin

Question 6

Recognise the structural components of a DNA and an RNA molecules

Answer 6

DNA:
Deoxyribose sugar, nitrogenous base (ATGC), phosphate
RNA:
Ribose sugar, nitrogenous base (AUGC), phosphate
Bases are purine (adenine, guanine) or pyrimidine (thymine, cytosine, uracil)
A-T 2 H bonds
G-C 3 H bonds

Question 7

Recognise and apply the conventions used to represent DNA/RNA base sequences

Answer 7

5’ATGC3’

3’TACG5’

Question 8

Explain polarity of a DNA or RNA chain

Answer 8

5’–>3’ (left to right)

Question 9

Explain the importance of hydrogen bonding and base pairing in defining nucleic acid secondary structure

Answer 9

A-T 2 H bonds
G-C 3 H bond
Hydrogen bonds are formed between anti-parallel, complementary sequences

Question 10

Describe the key features of the DNA double helix

Answer 10

Contains major and minor groove

Right handed helix of 10 bases per turn

Question 11

Explain how eukaryotic DNA is condensed in nucleosomes and relate this to the structure of chromosomes

Answer 11

Histone core (+ve)
Linker DNA (-ve)
Histones –> beads on a string –> solenoid
Heterochromatin - solenoid 30nm fibre (genes not expressed)
Euchromatin - beads on a string (genes expressed)