enzyme catalysts

Question 1

What is an enzyme?

Answer 1

Enzymes are usually proteins and facilitate chemical reactions under biologically relevant conditions.

Question 2

How many different classes of enzymes are there?

Answer 2

6

Question 3

What are the required conditions for enzyme working in a biological system?

Answer 3

• Low temperatures (37 in humans)

- Low reactant concentrations (10^-3 to 10^-6)

Question 4

What is a lyase?

Answer 4

an enzyme that cleaves C-C, C-O or C-N bonds by elimmination, leaving double bonds or rings. or that adds groups to double bonds/rings

Question 5

What is an isomerase?

Answer 5

an enzyme that catalyses the transfer of groups within molecules to yield an isomer as an end result

Question 6

What is a ligase?

Answer 6

an enzyme which catalyses the formation of C-O, C-C, C-S or C-N bonds by condensation reactions that are coupled to cleave ATP or a similar cofactor

Question 7

What is an active site?

Answer 7

the specific area of enzyme structure where the substrate binds and chemical groups of the enzyme facilitate catalysis of the reaction from substrate to product

Question 8

What is the equation for an uncatalysed reaction?

Answer 8

A+B —A.B—X—C.D–C+D

where X is the transition state

Question 9

What are the steps of enzyme working?

Answer 9

1. Old chemical bonds must break
2. New bonds must form
3. Complex in which neither original or final molecules are present (electrons are not forming bonds with product or reactant) = transition state
4. Energy is required to break old bonds to form transition state

Question 10

What is Gibbs free energy?

Answer 10

The potential energy of a chemical system

It is the energy bound up in the chemical bonds between atoms and their arrangement

Question 11

Describe the energetics of catalysis

Answer 11

1. Enzyme catalyst creates a modified transition state
2. The transition state has a much lower activation energy
3. This allows the reaction to proceed much more quickly
4. Overall reaction in terms of free energy will not change
5. Enzymes will not affect the position of equilibrium in the reaction

Question 12

What type of bonding occurs in substrate enzyme binding?

Answer 12

• result of non covalent interactions
• ionic bonds
• hydrogen bonding
• hydrophobic interactions

Question 13

What are the 3 major types of catalysis reactions?

Answer 13

• Acid base reactions
• stabilisation of charged intermediates
• providing highly reactive nucleophilic groups

Question 14

Provide an example of an enzyme and how it works

Answer 14

Chrymotrypsin
= His57 acts as a base to deprotonate Ser195
When the substrate is in the binding site, Ser195 can perform a nucleophillic attack
the oxyanion that is transiently formed in this reaction is stabilised by nearby delta positive hydrogen groups in the enzyme binding site
This is followed by a further electron rearrangement resulting in the breakage of the amide bond

Question 15

What is the function of the active site and it’s groups?

Answer 15

two main functions:

1. Binding of substrate
2. Catalysis

the function of AS groups is to add/remove H+ and to stabilise the charged transition state

Question 16

Compare reaction and substrate specificity

Answer 16

REACTION= enzymes will only catalyse a specific class of reactions

SUBSTRATE= enzymes exhibit varying degrees of substrate specificity
They can be very broad or very fussy

Question 17

What is the “lock and key” hypothesis?

Answer 17

A model where every enzyme has an active site into which the substrate fits precisely, like a key fits a lock. Substances that do not fit will not undergo a reaction

Question 18

What is the “induced fit” hypothesis?

Answer 18

A model where the active sit is not rigid and the shape of the active site alters when a substrate molecule binds at the sit. The substrate induces the correct fit of the active sit, the enzyme keeps the substrate under stress

Question 19

When are enzymes adapted to bind to substrate (induced fit hypothesis)?

Answer 19

Adapted to bind to the transition state between substrate and product, this facilitates the formation of the transition state, thus decreasing the activation energy of the reaction

Question 20

How does enzyme catalysis need to be measured in order to characterise enzyme function?

Answer 20

• substrate disappearance

- product formation

Question 21

What is Vo and Vmax

Answer 21

Vo= the initial velocity of an enzyme
Vmax = the maximum theoretical velocity of an enzyme

Question 22

What is the Michaelis- Menten equation?

Answer 22

Vo= Vmax/1+(Km/[S])

Question 23

What is Km?

Answer 23

the substrate concentration at half Vmax
in M (units)

Question 24

Desrcibe Km simplistically?

Answer 24

it is a measure of the affinity of the enzyme for its substrate. The higher the Km, the lower the strength of enzyme substrate bonding

Question 25

What is the lineweaver burk plot?

Answer 25

A graph which measures the inverse of Vo against the inverse of [S]

Question 26

What are the two classes of enzyme inhibitors?

Answer 26

irreversible and reversible

Question 27

What are irreversible VS reversible inhibitors?

Answer 27

irreversible = Combines with the enzyme covalently to give a permanently non reactive form of the enzyme

reversible = combine reversibly with enzyme, so that it regains full activity if an inhibitor is removed

Question 28

What are the 3 types of reversible inhibition?

Answer 28

competitive
uncompetitive
mixed (also uncompetitive)

Question 29

Describe competitive inhibition?

Answer 29

inhibitor binds to the active site but is not processed

Question 30

Describe uncompetitive inhibition?

Answer 30

inhibitor binds to a seperate site

inhibitor only binds to ES complex, not enzyme alone

Question 31

Describe mixed inhibition?

Answer 31

inhibitor binds to a site other than the active site
may bind to an E or ES complex
may not affect the binding of the substrate

Question 32

What happens to the lineweaver burk plot under competitive inhibition?

Answer 32

No change in Vmax as eventually, at high substrate conc. Vmax will still be reached

Increase in apparent Km as the efficiency of the substrate is decreased

Question 33

What happens to the lineweaver burk plot under non competitive inhibition?

Answer 33

Vmax decreases with increased inhibitor as the catalysis is being prevented
Km decreases as we are locking the enzyme substrate complex together

Question 34

What happens to the lineweaver burk plot under mixed inhibition?

Answer 34

Vmax decreases with increased inhibitor as catalysis is being prevented
Affects on Km are a combination of interference by inhibitor and locking ES complex

Question 35

What are allosteric enzymes characterised by?

Answer 35

1. Quaternary structure or multi domained enzymes
2. They have different kinetics to the normal model, they use sigmoidal kinetics
3. Substrate or modulator binding induces conformational change to facilitate substrate binding
4. Very flexible enzyme

Question 36

What are isoenzymes?

Answer 36

enzymes that catalyse the same reaction but have different primary structures

Question 37

Can isoenzymes be separated?

Answer 37

yes through physiochemical techniques, as they have different primary structures

Question 38

What is the equation for glucose phosphorylation?

Answer 38

ATP + Glucose = Glu 6-P + ADP

Question 39

Provide examples of enzymes used for clinical diagnosis of disease?

Answer 39

Creatine Kinase = different forms in skeletal muscle, heart and brain

Lactate dehydrogenase = pyruvate-lactate
LDH1- heart (myocardial infarction)
LDH5- liver (hepatitis)