enzymes

Question 1

exergonic vs endergonic reactions

Answer 1

exergonic = releases energy
energy required at start less than energy released by end = overall release

endergonic = require energy
energy required greater than released = overall need

Question 2

transition state

Answer 2

point of maximum energy requirement
needs to be reached before a reaction can occur

(top of graph)

Question 3

activation energy

Answer 3

energy required to make the reaction happen

molecules must collide before they can react
collisions don’t always provide enough energy for reaction to happen so sometimes an external source is needed eg heat

lower activation energy = reaction happens quicker

enzymes lower activation energy

Question 4

what is an enzyme?

Answer 4

globular proteins with a 3D tertiary structure

act as catalysts by lowering activation energy
provide an alternate transition state

reduces amount of energy needed to transform substrates into products
amount of energy released is the same just the reaction occurs quicker

Question 5

enzyme specificity

Answer 5

substrate and enzyme must collide and bind
only bind to specific substrates due to specific active site shape

active site composed of 2 parts:
substrate binding site
- complementary to substrate
- lock and key and induced fit

catalytic site

Question 6

types of specificity

Answer 6

absolute = only do one reaction on one substrate

bond = breaks specific bonds

group = enzymes that add specific groups to molecules

stereo - binding site binds specific parts of substrate in specific order

Question 7

catalytic mechanisms (5)

Answer 7

proximity - enzyme brings substate molecules close together so they can react

orientation - brings substrates to correct relative orientation

strain/distortion - enzyme binds to molecule putting strain on bond making it easier for reaction to occur

acid base catalysis - charges on enzymes means protons donated or accepted

covalent catalysis - temporary covalent bond formed between enzyme and substrate

more than one mechanism can be used to catalyse a reaction

Question 8

what is the turnover number?

Answer 8

number of substrate molecules that can be converted to products by 1 enzyme molecule in 1 second

Question 9

effect of time on enzyme action

Answer 9

initial reaction velocity (initial rate)

start occurs quickly as more substrate so a greater chance of collision
high substrate to enzyme ratio

rate slow as substrate decreases until plateau

reaches equilibrium
no net change in amount of substrate or product

Question 10

effect of pH on enzyme activity

Answer 10

change in pH effects electrostatic bonds in protein
changes 3D structure

cant bind and function properly
decreases enzyme activity

effects bonds in substrate so can’t bind = decreased activity

all have optimum pH where the work best
denatured in too high or low

Question 11

how does pH effect enzyme structure

Answer 11

decreasing pH
carboxylic acid soup gains proton, loses negative charge and electrostatic bond broken

increasing pH
NH3+ loses proton, positive charge lost, bond broken

also effects charged groups and therefore bonds in substrate
alters shape and therefore decreased activity

Question 12

effect of temperature on enzyme activity

Answer 12

increasing temperature = more energy
kinetic effect

molecules move and vibrate more
number of collisions increases
= increased rate of reaction

until certain temperature
= denature

vibrate too much and breaks bonds between molecules
may reform elsewhere
= shape changed

Question 13

effect of substrate concentration on enzyme activity

Answer 13

if number of enzyme molecules remain constant,

low concentrations
substrate concentration determine rate of reaction
- not enough collisions, active sites not full so rate slowed

high concentrations
number of enzyme molecules determine rate
- not enough enzymes for substrate to bind to, active sites full so rate limited

Question 14

first order kinetics

Answer 14

velocity directly proportional to substrate concentration
velocity dependant on substrate

Question 15

zero order kinetics

Answer 15

velocity independent of substrate
number of enzyme molecules determine velocity instead

Question 16

hyperbolic curve

Answer 16

typical curve on graph of typical enzymes
Michaelis-Menten kintetics

Question 17

Michaelis-Menten kinetics

Answer 17

Km = indicator of affinity of an enzyme for its substrate

high Km = low affinity
- ES complex less stable
- enzyme doesn’t bind to substrate well
- more substrate needed to stabilise therefore high Km

low Km = high affinity
- ES complex stable at lower concentrations
- enzyme binds to substrate well

plateau on graph only reached at very high concentrations
hard to reach in lab conditions
so Lineweaver-Burke used

Question 18

what is Vmax?

Answer 18

maximum rate of reaction

where curve plateaus
determined from mm of curve

Question 19

what is the Michaelis constant (Km)

Answer 19

substrate concentration at half Vmax (maximum rate of reaction)

Question 20

Lineweaver Burke

Answer 20

plot the reciprocals of Vmax and Km
produces a straight line that can be extrapolated

however prone to high error
hard to accurately determine concentration when working at low concentrations

Question 21

Lineweaver-Burke equations

Answer 21

gradient = Km over Vmax

1/Vmax = line intercepts Y axis

-1/Km = extrapolated back to x axis

Question 22

what is an inhibitor?

Answer 22

molecules that reduce the rate of an enzyme catalysed reaction

can be irreversible or reversible
reversible = competitive and non-competitive

Question 23

irreversible inhibitors

Answer 23

binds irreversibly to enzyme
form strong covalent bond
bind to amino acid side chain near active site
- commonly serine or cystine

inactivates enzyme by preventing substrate binding

Question 24

reversible inhibitors

Answer 24

can be removed
includes

competitive
- bind to active site and compete with substrate

non-competitive
- bind to another site

Question 25

what is a competitive inhibitor? (r)

Answer 25

compete for access to active site similar structure to substrate prevents binding of substrate - doesn't prevent catalysis can be overcome by increasing substrate concentration = outcompete inhibitor (more likely to collide)

Question 26

effects of competitive inhibitors

Answer 26

Km increases forms enzyme inhibitor complex therefore less ES complexes amount of substrate required to reach half Vmax increases more substrate needed to outcompete and form ES not EI complexes Vmax remains the same maximum rate unchanged increase substrate concentration to outcompete inhibitor always reach same Vmax despite inhibitor

Question 27

what is a non-competitive inhibitor? (r)

Answer 27

bind at site other than active site can occur before or after substrate binds cont prevent substrate binding - but prevent catalysis cant be overcome by increasing substrate concentration instead repeat dialysis to remove enzyme inhibitor substrate complex made and changes shape of active site binding of substrate not affect but cant catalyse reaction

Question 28

effects of non-competitive inhibitors

Answer 28

Km remains the same doesn't effect binding of substrate amount required to reach half Vmax remains the same Vmax decreases prevent catalytic act maximum rate of enzyme decreased

Question 29

enzyme activators

Answer 29

increase rate of reaction or needed for activity of enzyme also called cofactors eg coenzymes coenzyme A or acetyl CoA

Question 30

allosteric enzymes

Answer 30

composed of 2 or more subunits subunit can be in active or inactive form active = low affinity for substrate inactive = high affinity for substrate one subunit binds to substrate initiates change of structure, stabilises active form (high affinity) substrate then more easily binds to second subunit as stabilised in active not inactive - co-operativity create sigmoidal kinetics (not hyperbolic)

Question 31

control of allosteric enzymes

Answer 31

each substrate has one or more effector binding sites positive effector = allosteric activators - increases overall enzyme activity bind to effector binding sites stabilise active form, substrate binds more readily increased activity negative effector = allosteric inhibitor - decrease overall enzyme activity bind on effector sites, stabilise inactive form substrate binds less readily decreased activity

Question 32

sigmoidal curve

Answer 32

s shaped curve lag phase move from inactive (low affinity) to active (high) form substrate binds to subunits and changes shape allows substrate to bind more easily

Question 33

effect of allosteric enzymes on sigmoidal curve

Answer 33

positive effector curve moves left Vmax reached quicker increase in activity over substrate concentration quicker negative effector curve more right vmaz reached slower increase in activity over substrate concentration slower