Enzymes

Question 1

what are enzymes?

Answer 1

globular proteins that act as catalysts for biological reactions

Question 2

how do enzymes differ from chemical catalysts? (4)

Answer 2

• they can catalyse very high reaction rates
• are highly specific
• work well at mild pH/temperatures
• can be regulated

Question 3

what are ribozymes?

Answer 3

catalytic RNA compounds with no protein component

Question 4

what are co-factors?

Answer 4

a non-protein component of an enzyme that is crucial to its activity (tend to me metal ions)

Question 5

what are coenzymes?

Answer 5

complex organic molecules that are usaully produced form vitamins and help to enhance an enzymes activity

Question 6

what are prosthetic groups?

Answer 6

cofactors covalently bonded to the enzyme or very tightly associated with the enzyme

Question 7

what is an apoenzyme?

Answer 7

the protein component of an enzyme that contains a prosthetic group

Question 8

what is a holoenzyme?

Answer 8

the “whole” enzyme, i.e apoenzyme plus the cofactor/s

Question 9

what ending is usually used when naming enzymes?

Answer 9

• ase

* name usually refers to function

Question 10

what are the 6 classes of enzymes?

Answer 10

• oxidoreductases
• transferases
• hydrolases
• lysases
• isomerases
• ligases

Question 11

what do oxidoreductases do?

Answer 11

transfer electrons

Question 12

what do transferases do?

Answer 12

transfer groups

Question 13

what do hydrolases do?

Answer 13

hydrolysis

Question 14

what do lysases do?

Answer 14

form or add groups to double bonds

Question 15

what do isomerases do?

Answer 15

transfer groups within a molecule

Question 16

what do ligases do?

Answer 16

formation of C-C, C-S, C-O and C-N bonds

Question 17

what do enzymes do? (3)

Answer 17

• increase rate of spontaneous reactions
• lower the activation energy of biochemical reactions
• accelerate movement towards reaction equilibrium

Question 18

how does an enzymes structure allow it function?

Answer 18

they have active sites complimentary to the transition state of the substrate/s

Question 19

what is binding energy?

Answer 19

the energy released when enzymes form non-covalent bonds with the substrate/s

Question 20

what do reactants need to break in order to react?

Answer 20

the energy barrier

Question 21

how do enzymes work

Answer 21

• reduce entropy (bring reactants closer together)
• desolvation (substrate-enzyme bond overcome H-bonds)
• induced fit

Question 22

what is Vmax?

Answer 22

the maximum rate of reaction that occurs when all the active sites have been filled

Question 23

what equation can be used to analyse reaction kinetics?

Answer 23

the michaelis-menten equation

Question 24

what does the michaelis-menten reaction deuce about reaction speeds?

Answer 24

the breakdown of ES to E + P is slower than the formation of ES form E + S

Question 25

what is another technique (graphical) used to analyse reaction kinetics?

Answer 25

Lineweaver-Burk plot

Question 26

what does the x-interecpt and y-intercept show on a lineweaver-burk plot?

Answer 26

``` x-intercept = 1/Km y-interecpt = 1/Vmax ```

Question 27

what is the definition of Km?

Answer 27

the ratio of rate constant of breakdown of ES to E+S cmpared to the rate constant of the formation of ES from E+S

Question 28

what does a high Km show?

Answer 28

a less stable ES complex i.e low affinity between enzyme and substrate

Question 29

what are isozymes?

Answer 29

enzymes that catalyse the same reaction

Question 30

what are the properties and location of glucokinase?

Answer 30

- high Km (for glucose) - high Vmax - found in liver

Question 31

what properties and location does hexokinase show?

Answer 31

- low Km - Low Vmax - found in all body cells

Question 32

what happens to glucokinase activity when blood glucose drops and why is this useful?

Answer 32

glucokinase activity drops allowing glucose released by glucokenesis to leave the liver (as it isn't converted to glucose-6-phosphate)

Question 33

what reaction do glucokinase and hexokinase catalyse?

Answer 33

glucose + ATP ----> glucose-6-phosphate + ADP

Question 34

what do increased levels of intracellular enzymes in plasma indicate?

Answer 34

tissue damage resulting from trama, necrosis etc

Question 35

what technique can be used to separate different forms of enzymes?

Answer 35

gel electrophoresis

Question 36

how many forms of creatine phosphate are there?

Answer 36

3 forms (CK1, CK2 and CK3)

Question 37

what is a ternary complex?

Answer 37

a complex which holds all of the reacting substrates and enzyme together

Question 38

what are the diffrent types of reaction mechansims for enzyme catalysed reactions with more than one substrate?

Answer 38

- random ordered with a ternary structure - ordered with a ternary structure - no ternary structure formed (sequential)

Question 39

what type of mechansim does lactose dehydrogenase use?

Answer 39

an ordered sequential mechanism with a ternary structure | - NADH + pyruvate ----> lactate and NAD+

Question 40

describe the mechansim of creatine kinase

Answer 40

random sequential mechansim with a ternary structure | - 2x ATP + 2x Creatine -----> 2x Phosphocreatine + 2x ADP

Question 41

describe the mechansim between amino acids and ketoacids

Answer 41

double displacement (ping-pong) mechanism in which the substrates bounce on the enzyme and then off again once they've reacted, meaning no ternary structure forms

Question 42

what are allosteric enzymes?

Answer 42

enzymes made up of many sub-units, all of which contain active sites

Question 43

what factors affect enzyme activity?

Answer 43

- pH - temperature - inhibitors

Question 44

what are the different types of inhibitors and how do they work?

Answer 44

COMPETITIVE: resemble substrate and bind non-covalently to active site (lowering Km) NON-COMPETITIVE: bind to part of enzyme that's not the active site non-covalently (lowers Vmax) UNCOMPETITIVE: inhibitor binds above active site after the substrate has bound, preventing the product molecules form being released

Question 45

give an example of a competitive inhibitor and explain how it functions?

Answer 45

AZT | - inhibits reverse transcriptase as it is an analogue to a DNA precursor TTP

Question 46

what are irreversible inhibitors?

Answer 46

inhibitors that bind to enzymes covalently | - e.g cyanide which binds to cytochrome c oxidase disrupting terminal respiration, starving cells of ATP

Question 47

what are the two types of regulatory enzymes?

Answer 47

allosteric enzymes and covalently modified enzymes

Question 48

give an example of feedback inhibition

Answer 48

threonine dehydrogenase which is inhibted by the final product in a pathway; l-isoleucine - l-isoleucine binds to enzyme causing a confromational change that blocks the enzymes activity

Question 49

what is feedback inhibition?

Answer 49

a build of the end product in a pathway that slows the entire pathway by altering the enzymes activity

Question 50

what are allosteric effectors?

Answer 50

cell metabolites that bind non-covalently to part of an enzyme (non-competitive) chnaging the enzymes activity - can be activators or inhibitors

Question 51

explain the concerted model

Answer 51

allosteric enzymes have two confromations that they randomly switch between, one conformation binds the substrate well, the other doesnt - the binding of substrate locks the enzyme in an "open" conformation allowing other substrates to bind

Question 52

what shape of curve do allosteric enzyme kinetics show?

Answer 52

a sigmoid shape

Question 53

explain the sequential model

Answer 53

there is no flipping between confromations, instead the binding of a substrate causes a conformational change that "opens" the next sub-unit allowing a second substrate molecule to bind and so on

Question 54

what is covalent modification?

Answer 54

modification of enzyme activity through covalent bonding (often involves transfer of groups) e.g phosphorylation

Question 55

what do protein kinases do?

Answer 55

add phosphoryl groups

Question 56

what do protein phosphatases do?

Answer 56

remove phosphoryl groups

Question 57

what is proteolytic cleavage?

Answer 57

the cleavage of an inactive precursor protein (proenzyme or preprotein) to give an active protein/enzyme e.g production of digestive enzymes (zymogens)