Enzyme Regulation

Question 1

What is enzyme kinetics

Answer 1

Study of rate of enzyme catalyse reactions, and how the rate varies with different substrate concentrations/co factors/inhibitors/ metal ions/ ph

Question 2

How does reaction rate vary with substrate concentration

Answer 2

First order at Low substrate conc,

Zero order at high substrate conc

Question 3

What’s the michaelis menton reaction model, and what are three assumptions about it

Answer 3

E + S ES -k2-> E + P

1. [S] > [E] so amount of substrate bound at any one time is small
2. [ES] does not change with time. Formation of ES = breakdown of ES
3. Initial velocities used, concentration of product small and back reaction can be ignored

Question 4

What’s the michaelis menton equation, and what happens when v0= 0.5v max

Answer 4

V0= (Vmax[S])/(Km + [S])

Km= (K2 + K-1)/K1

When V0=0.5 Vmax, Km=[S]

Question 5

When does Km represent substrate affinity?

Answer 5

When K2 is limiting (K2<

Question 6

What is kcat?

Answer 6

Turnover number = number of substrate converted to product in a unit of time when enzyme is saturated with substrate

Question 7

Lineweaver burke plot equation

Answer 7

1/V0 = (Km/{Vmax[S]}) + 1/Vmax

Question 8

4 monomers of lactate dehydrogenase

Answer 8

H4, M4, H3M, H2M2, HM3

Question 9

Is competitive inhibition reversible? Example

Answer 9

Yes

Malonate inhibition of succinate dehydrogenase

Question 10

Examples of non competitive inhibition (reversible and irreversible)

Answer 10

Reversible: EDTA inhibits Mg2+ requiring enzyme

Irreversible: organophosphorus inhibition of cholinesterase

Question 11

How do competitive inhibitors affect Km and Vmax?

Answer 11

Apparent Km is larger, Vmax remains the same

Question 12

How do non competitive inhibitors affect Km and Vmax

Answer 12

Vmax is lowered, Km remains the same

Question 13

How does angiotensin work? And what do ACE inhibitors do

Answer 13

Heart disease -> decreased tissue perfusion-> decreased renal flow-> decreased glomerular filtration-> reduced [Na] in distal tubule -> renin released -> angiotensinogen is converted to angiotensin 1(by cleavage between L & L) -> ACE converts angiotensin 1 to angiotensin 2 (cleavage between (H & F) ->
Aldosterone release-> water reabsorption-> oedema
Peripheral vasoconstriction
Increased blood volume-> high bp-> heart disease

ACE inhibitors inhibit ACE

Question 14

What’s an acetylcholinesterase inhibitor?

Answer 14

inhibits the acetylcholinesterase enzyme from breaking down acetylcholine
Used in Alzheimer’s

Question 15

How Is enzyme activity regulated

Answer 15

1. Allosteric binding sites
2. Covalent modification
3. Induction or repression of enzyme synthesis

Question 16

What effect do allosteric enzymes have on the reaction velocity VS [S] curve? Positive effectors vs negative effectors
Examples

Answer 16

Normal enzymes show hyperbolic curve
Allosteric enzymes show a sigmoid curve.
Positive effectors bind outside active site and make it more complementary to substrate
Negative effectors bound outside active site and make it less complementary to substrate

-ve: ATP and citrate on phosphofructokinase
+ve: phosphoenolpyruvate (PEP) and fructose 1,6 bus phosphate on pyruvate kinase

Question 17

Allosteric regulation of phosphofructokinase

Answer 17

Fructose 6 phosphate is converted by phosphofructokinase to fructose 1,6 bisphosphate

AMP activates the inhibited PFK
ATP/ citrate inhibits PFK

Question 18

Allosteric regulation of pyruvate kinase

Answer 18

PEP is converted to pyruvate by pyruvate kinase.

Fructose 1,6 bisphosphate and PEP are positive effectors

Question 19

5 methods of covalent modification

Answer 19

1. Phosphorylation
2. Adenylylation
3. Uridylylation
4. ADP-ribosylation
5. Methylation

Question 20

What happens in phosphorylation and dephosphorylation?

Answer 20

Phosphorylation may result in increased/decreased activity

Addition /removal of phosphate from S/T/Y/H
Increased activity of glycogen phosphorylase by phosphorylation by phosphorylase kinase serine 14
Dephosphorylation by phosphorylase phosphatase

Phosphorylation at ser residues by glycogen synthase kinase 3-> Decreased activity of glycogen synthase
Dephosphorylation by phosphoprotein phosphatase

Phosphorylation at several Ser residues (by glycogen synthase kinase 3) inactivates enzyme

Dephosphorylation by a phosphoprotein phosphatase

Question 21

How blood glucose levels regulate enzymes

Answer 21

High blood glucose-> elevated insulin-> increased synthesis of glucokinase/ phosphofructokinase / pyruvate kinase