Enzyme regulation

Question 1

What is a metabolic pathway?

Answer 1

The enzymatic route that a metabolite (any molecule that is acted on by an enzyme and is turned into something else) molecule takes

There’s always faster enzymes vs slower so there needs to be regulation so this doesn’t fall apart

Question 2

What are rate limiting steps?

Answer 2

1. Slowest reaction in pathway
2. Determines overall flux (flow of molecules from one step to the next) through the pathway by acting as a bottleneck (natural diversion)
3. usually the first unique enzyme in a pathway. In a branched pathway, it’ll occur after a branch so products can be made independently of each other

Question 3

How to identify RLS?

Answer 3

1. Measure Vmax for enzymes in pathway. Lowest Vmax = RLS
2. if non of enzymes work at their normal speeds then compare equilibrium constant and mass action ratios (MAR are a measure of what’s going on in the cell):

If reaction equillibrum then MAR = Keq approx

If reaction is rate limiting MAR < Keq (usually MAR is 100-1000 times < Keq)

3. Test if step is at crossover point (a change to the system which changes the pathway so allows to identify?) DIAGRAM IN ON.E.g glycolysis is faster in anaerobic conditions so RLS in glycolysis change their affect as a result of going anaerobic

Question 4

How to work out MAR (mass action ratio)

Answer 4

[C][D]/[A][B]

Question 5

What’s the RLS in Glycolysis? Explain this.

Answer 5

Hexokinase. Lowest Vmax, Mar < Keq, gets slower in response to going anaerobic

Question 6

What is futile cycling?

Answer 6

occurs when two metabolic pathways run simultaneously in opposite directions and have no overall effect other than wasting energy in the form of heat. This is done to warm up the body (e.g bumblebees do this)

Question 7

Types of enzyme regulation

Answer 7

1. Instrinsic control by metabolites - allosterism/inhibitors - changes in the cell cause the cell to alter their regulation.
2. fast extrinsic control - cell is told to change metabolism for other purpose (usually hormones do this) - covalent modification e.g phosphorylation
3. slow extrinsic control - gene expression - change gene expression within cell to make more or less enzyme

Question 8

What does Fructose 2,6-bisphosphate do?

Answer 8

Produced when levels of F-6-P gets high

Allosterically activates PFK-1 to activate the F-6-P to F-1,6-P2 reaction (re-activating it after inhibition bc too much F-6-P) DIAGRAM IN ON

Also inhibits F-1,6-bisphosphatase reverse reaction.