Kinetics

Question 1

which enzymes catalyze glucose –> glucose 6 phosphate?

and for which pathways

and where are they found

Answer 1

hexokinase (glycolysis- breakdown glucose) (most tissue)

glucokinase (glycogenesis-store glucose) (liver)

Question 2

apply Vmax and Km to glucokinase and hexokinase

Answer 2

lower vmax, lower km (higher affinity), turned off by high [ ] of glucose 6-phosphate

–>product inhibition with hexokinase

glucokinase has higher km, lower affinity than hexokinase in a fasting state

Question 3

which body states are hexokinase and glucokinase active in?

Answer 3

hexokinase is active during fasting (glycolysis - use glucose for energy)

glucokinase is active after a high-carb meal (glycogenesis- store as glycogen)

glucokinase has higher km, lower affinity than hexokinase in a fasting state

Question 4

glucokinase and the liver

Answer 4

nutrients absorbed from intestine go here 1st–> allow glucokinase to store excess glucose from a meal as glycogen

Question 5

Does glucokinase (glycogenesis- store as glycogen) have a high or low vmax after a meal?

Answer 5

high vmax= high capacity to convert substrate –> product

glucokinase can phosphorylate lots of glucose to glucose 6-phosphate after a meal and store as glycogen

Question 6

hexokinase vs glucokinase

Answer 6

hexokinase= for energy, stops until you eat, inhibited by glucose 6 phosphate

glucokinase= for storage, high vmax, lots of glucose –> glucose 6-phosphate, continues as long as glucose is high, shuts off when glucose is low bc low affinity

Question 7

Km and Kcat

Answer 7

-enzyme efficiency
-large Kcat, small Km= efficient

equation: kcat/ km

kcat= measures speed of P formation once ES has been made
km= measures binding affinity of E and E to make ES

E + S <–> ES –> E + P

Question 8

what are the 3 types of enzyme inhibition

Answer 8

1. reversible inhibition (competitive, uncompetitive, noncompetitive)
2. irreversible inhibition
3. inhibition of multi-subunit allosteric enzymes

Question 9

reversible inhibition- competitive inhibition

Answer 9

reversible binding of inhibitor to enzymes active site
-compete with substrate for its spot

-vmax: unchanged bc add more substrate will kick off inhibitor- doesn’t effect enzyme activity

-km: increases bc lower affinity, need more substrate to kick off inhibitor and bind

Question 10

reversible inhibition- uncompetitive inhibition

Answer 10

reversible binding of I (inhibitor) to ES (enzyme substrate complex)

-inhibitor binds enzyme ONCE ES complex formed

-vmax: decreases, inhibitor prevents ES from completing rxn
-km: decreases, more affinity, ES cant dissociate

Question 11

reversible inhibition- noncompetitive inhibition

Answer 11

reversible binding of I to E OR ES
-i.e. product inhibition: glucose 6-phosphate inhibits hexokinase

-vmax: decreases bc amount of enzyme present is reduced
-km: unchanged- binding to E decreases affinity, binding to ES increases affinity so it cancels out

Question 12

compare the Km and Vmax changes for reversible inhibition (competitive, uncompetitive, noncompetitive)

Answer 12

competitive: no change in vmax, increase km

uncompetitive: decrease vmax, decrease km

noncompetitive: decrease vmax, no change in km

Question 13

irreversible inhibition

Answer 13

when inhibitor forms covalent bond with active site of enzyme (i.e. penicillin, lead poisoning)

Question 14

inhibition of multi-subunit allosteric enzymes

Answer 14

add substrate, increases rxn rate and changes shape of active site

-sigmoidal graph: inhibitor produces a right shift in this S shaped graph (need more [s] for for enzyme to work and reach vmax)

i.e. PFK1 (multisubunit)
-when 1 F6P binds, enhances binding of more F6P to other subunits
-allosterically inhibited by ATP - turn off enzyme activity when at high [ ]