enzymes

Question 1

what is the general role of enzymes

Answer 1

to catalyse biological reactions by lowering the activation energy

Question 2

what does delta G have to do with reactions

Answer 2

-ve delta g means the reaction will occur spontaneously
+ve delta g means the reaction will occur non spontaneously

Question 3

do enzymes alter the equilibrium of a reaction?

Answer 3

they do not alter their equilibrium concentrations

Question 4

what are the 6 major classes of enzymes

Answer 4

1) oxoreductase
2) transferase
3) hydrolase
4) lyase
5) isomerase
6) ligase

Question 5

what is a cofactor (2 classes) and what is its purpose

Answer 5

metal ions and co enzymes
these are other non protein factors that help to catalyse reactions

Question 6

how do metal ions act as a co-factors

Answer 6

they act as lewis acids ( electon accepting) and can participate in acid base catalysis.
they form coordination compounds with precise geometrics, good for positioning reactants exactly where they’re needed.

Question 7

how do co-enzymes act as co-factors?

Answer 7

They’re small organic molecules, they work as carriers ( atoms,electrons, functional groups etc)

Question 8

what is reaction coupling and how do enzymes mediate this

Answer 8

enzymes couple spontaneous reactions to non spontaneous ones, overall making delta G -ve = a spontaneous reaction.

Question 9

what are 3 features of an enzymes active site

Answer 9

they have amino acid side chains projecting into it
they bind substrates via multiple weak interactions
these determine the specificity of the reaction

Question 10

what are the 4 types of enzyme substrate bonds and what is a feature of the bond

Answer 10

ionic- non covalent bonds with charged side chains
hydrogen bonds- useful for specific geometry, side chaisn or backbone often act as H+ bond donors/acceptors
van der waals- weakest of the bonds. Between protein and substrate atoms in close proximity
covalent bonds- rare bonds, much stronger. only occur if you can reverse to original state.

Question 11

what does the induced fit model demonstrate

Answer 11

that enzymes are dynamic, active site side chains can be subtly rearranged to create a network of H+ bonds at the active site to best fit the substrate

Question 12

what is covalent catalysis

Answer 12

a reactive, short lived intermediate that is covalently attatched to the enzyme.

Question 13

what type of amino acids are likely to be involved in acid base catalysis

Answer 13

glu, asp- positively charged
lys- arg- negatively charged
histidine particularly suitable as its pKa is close to physiological PH so it doesn’t take much to make it a proton donor or acceptor.

Question 14

What is expected to bind tightest to an enzyme: the substrate, product or transition state?

Answer 14

should bind more tightly to transition state then the substrate.

Question 15

how is the progression of a reaction through the tranisiton state affected by the presence of an enzyme

Question 16

what are the two models used to describe enzyme- substrate binding

Answer 16

lock and key model- enzyme and substrate fit perfectly into eachother
induced fit model- simular but small changed are made to induce the fit once they join together,

Question 17

how is activation energy lowered?

Answer 17

1)ground state destabilisation- dont want the enzyme substrate complex to be too comfortable that it doesnt want to react

2)transition state stabilisation- by reducing the activation energy the transition state becomes more energetically favourable

3) Alternative reaction pathway- different energy transition states.

Question 18

what are the three main catalytic pathways that enzymes are involved in?

Answer 18

proximity and orientation effects, preferential binding of transition state, acid- base catalysis

Question 19

when there is an excess of substrate, reaction velocity is _______ to enzyme concentration

Answer 19

proportional

Question 20

what does the Y intercept represent on a lineweaver- burk plot?

Answer 20

1/Vmax

Question 21

what does the X intercept represent on a lineweaver- burk plot?

Answer 21

-1/ Km

Question 21

what is the significance of Km

Answer 21

it tells us how well the enzyme grabs the substrate
it measured by the substrate needed to reach 1/2 of Vmax.

Question 22

what does a high or a low Km tell us

Answer 22

high= the enzyme has a low affinity for the substrate
low= the enzyme has a high affinity for the substrate

Question 22

if an enzyme has multiple substrates, how can we use Km to determine substrate preference?

Answer 22

The lower the Km value the more preference the enzyme has for that particular substrate.

Question 23

what does Kcat for an enzyme catalysed reaction represent?

Answer 23

The number of substrate molecules converted to product per enzyme per unit of time when E is saturated with substrate.
the activity of one enzyme molecule= a measure of catalytic activity

Question 24

how do we define catalytic efficiency?

Answer 24

peak enzyme should have
high Kcat - large turnover of substrate into products per second
low Km- high affinity for the substrate

Kcat/Km= overall measure of enzyme efficiency

Question 25

what 3 assumptions can be made to simplify a michaelis- menten model reaction?

Answer 25

1) product isnt converted back into substrate
2) Haldones steady stade: rate of enzyme substrate formation is equal to its breakdown
3) measuring initial rates means that substrate concentration doesnt change significantly

Question 26

what are the two main classes of inhibitors and how do they differ in their binding to an enzyme?

Answer 26

Irreversible- binds to the enzyme and permanently inactivates it.
Reversible- binds to the enzyme but is later released

Question 27

How can competitive inhibition of an enzyme be overcome?

Answer 27

Adding infinite substrate can outcompete the inhibitor

Question 27

What effect does a competitive inhibitor have on Km and Vmax in an enzyme catalysed reaction?

Answer 27

the inhibitor competes directly with the substrate.
no change in Vmax or Km.

Question 28

Where on an enzyme does a competitive inhibitor bind?

Answer 28

the enzymes active site

Question 29

what effect does a pure non-competitive inhibitor have on Km and Vmax in an enzyme catalysed reaction?

Answer 29

Vmax decreases but Km stays the same

Question 30

where on an enzyme does a non-competitive inhibitor bind?

Answer 30

Inhibitor binds at a different site than the substrate

Question 31

What is mixed non-competitive inhibition and what effect does it have on Km and Vmax in an enzyme catalysed reaction?

Answer 31

when the inhibitor binds the enzyme whether or not the enzyme has already bound the substrate.
Mixed inhibiton causes both Vmax and Km to change

Question 32

what are allosteric enzymes?

Answer 32

Things that happen away from the active site of an enzyme