Enzymes I

Question 1

what are enzymes?

Answer 1

proteins that catalyze specific chemical reactions by lowering the activation energy required and facilitating the formation of a transition state which is usually unstable

Question 2

What are some functions of proteins?

Answer 2

→DIGESTION: carbs, fats, proteins

→BLOOD CLOTTING: fibrin clot, activation by thrombin

→DEFENCE: immune system, activation of complement

→ MOVEMENT: muscle actomyosin is an ATPase

→NERVE CONDUCTION: membrane ion pumps for Na+, Ca2+

Question 3

what are two diseases caused by enzyme defects?

Answer 3

→Phenylketonuria - phenylalanine cannot be converted into tyrosine.

Tay sachs cerebroside cannot be made

Question 4

How do enzymes get used as drug targets?

Answer 4

penicillins inhibit cell wall synthesis in bacteria (murein)

aspirin blocks prostaglandin

methotrexate has a similar shape to folic acid which is needed to make dNTPs so it blocks an enzyme that synthesizes dNTPs/

Question 5

where does evidence for active sites come from?

Answer 5

X-ray crystallography and kinetic studies

Question 6

what is the lock and key model?

Answer 6

the substrate is directly complementary to the enzyme and fits like a lock in a key

Question 7

what is the active site?

Answer 7

→a 3D cavity that binds substrates using electrostatic, hydrophobic, hydrogen and van der waals interactions

Question 8

what is the induced fit model?

Answer 8

the enzyme is flexible and as the substrate starts to bind the active site changes shape to fit it more closely

Question 9

what are the factors responsible for enzyme catalysis?

Answer 9

APPROXIMATION : the enzyme can bring together two reactants
and it can strain the bond.
COFACTORS can be bound to the active site to change the chemistry of the reaction.
The enzyme can also neutralize positive and negative charges and exclude solvent from the reaction site.

Question 10

what is Vmax

Answer 10

the maximum possible velocity of the reaction when all the active sites are occupied

Question 11

what is Km a measure of?

Answer 11

stickiness of the active site for the substrate

Question 12

what does a high Km mean?

Answer 12

a lot of substrate is needed to bind to the active site (high concentration) so the affinity for the substrate is low

Question 13

why is a lineweaver burke plot more accurate? (linear form of the graph of velocity vs concentration)

Answer 13

V max does not have to be estimated it can be calculated

Question 14

how do you find the turnover number?

Answer 14

V max divided by the number of enzymes.

Question 15

what is the turnover number?

Answer 15

the maximum number of chemical conversions of substrate molecules per second that a catalytic site will execute for a given enzyme concentration

Question 16

what happens to v max and Km during non competitive inhibition?

Answer 16

→the inhibitor binds to the allosteric site.

→V max is decreased but km remains unchanged.

→Less active sites available but the affinity has not changed.

Question 17

what happens to Km and Vmax during competitive inhibition?

Answer 17

→V max is the same but Km is increased,

→the reaction can still achieve maximum velocity but you need a lot more substrate to achieve that value.

Question 18

what are the mechanisms for enzyme regulation?

Answer 18

→ALLOSTERIC : lac operon

→COMPARTMENTATION : a sequence that helps enzymes get to their final destination

→PHOSPHORYLATION : tyrosine kinase can activate or inactivate an enzyme by phosphorylating it.

→GENE EXPRESSION : controlling the amount of enzyme made

Question 19

what are properties of allosteric enzymes?

Answer 19

→multi subunit complexes
→regulatory sites and catalytic sites are on different subunits
→regulation happens via conformational changes
→involved in feedback inhibition
→non michaelis menten kinetics

Question 20

what are key enzyme properties?

Answer 20

→they increase the reaction rate up to 10 billion fold

→ they show specificity (only catalyse certain reactions)

→they’re unchanged at the end of the reaction

→ they don’t alter the reaction equilibrium

→ they facilitate the reaction by decreasing the free energy of the activation of the reaction

Question 21

What is enzyme-substrate binding energy used for?

Answer 21

→to bring molecules together in the active site

→ to constrain substrate movement -

→ to stabilise the positive and negative charges int he t-state -

→to strain particular bonds in the substrate, making breakage easier (the substrate is distorted on binding to resemble the transition state) -

→to use cofactors (they bring new chemistry to the active site)

Question 22

what is feedback inhibition?

Answer 22

→substance A is converted through a number of steps to Z.

→If we want to regulate this pathway, we can do this through feedback inhibition, which is as the product builds up, this inhibits the reaction of A, so this is done by allosteric regulation