Regulation of protein activity

Question 1

give two examples of isoenzymes and how they regulate protein activity

Answer 1

glucokinase and hexokinase

different kinetic properties with different aa sequences, vmax and km values.
hexokinase lower km than glucokinase

control the rate of glucose ultilisation in different tissues.

Question 2

give four ways in which substrate and product concentration affect rate

Answer 2

substrate availability
isoenzymes
availability of coenzymes e.g. NAD and NADH
product inhibition - glucose 6 phosphate inhibits hexokinase

Question 3

what shape is the graph between substrate concentration and reaction rate for allosteric enzymes?

NB why cannot you talk about km?

Answer 3

sigmoidal

not hyperbolic so cannot refer to km

Question 4

give two features of allosteric enzymes

Answer 4

multisubunit
more than one active site

binding of substrate to one A.S. enhances substrate binding to other active sites - creates sigmoidal relationship

positive cooperatively
T and R state

Question 5

Give features of allosteric activators

Answer 5

bind outside AS
changes conformation of one subunit so changes over 3D shape to enhance binding of substrate
increases affinity for substrate =
increases rate
binds to r form and stabilises - increasing proportion of enzyme in r state

shifts curve to LEFT

amp on glycogen phosphorylase

Question 6

give an example of an allosteric activator

Answer 6

AMP

fructose 2,6 bisphosphate

on phosphofructokinase

Question 7

describe the action of allosteric inhibitor

Answer 7

increases the proportion of enzyme in the T state

(shifts r to t conformational equilm towards t whereas allosteric activator shifts towards right)

shifts curve to RIGHT

Question 8

give an exmaple of allosteric inhibitor

Answer 8

ATP

citrate and H+

on phosphofructokinase

Question 9

how can substrate concentration reuglate enzymes?

Answer 9

increase conc of substrate = increase in rate

Question 10

Give an example of covalent modification of proteins

Answer 10

phosphorylation - adding phosphate group from ATP onto aa with hydroxyl groups Ser, threonine and tyr
adds bulky charged group - changes enzyme confomraiton/ substrate binding
protein phosphatase’s catalyse hydrolytic removal of phosphoryl groups.

Question 11

how do kinases cause enzyme cascades?

Answer 11

kinases can phosphorylate on three enzymes which can each further activate three more enzymes

amplification of signals by kinase cascades allows amplification of the initial signal by several orders of magnitude very quickly

Question 12

define zymogen

Answer 12

the inactive precursor of a proteolytic enzyme