Enzymes 2

Question 1

Define ‘isoenzyme’

Answer 1

Different enzymes that catalyse the same reaction

Question 2

Examples of isoenzymes include?

Answer 2

Glucokinase (Hexokinase IV) and Hexokinase I

Question 3

How can enzymes tell us about disease?

Answer 3

Finding them in the wrong place can be indicative of different pathologies

Question 4

Increased intracellular enzymes seen in the blood plasma is indicative of what?

Answer 4

Damage to cells (necrosis) due to trauma or disease

Question 5

How is normal activity displayed in a clinical setting?

Answer 5

Samples are described in terms of arbitrary units to make it easy to spot an abnormality

Question 6

Most enzyme reactions don’t follow a _____ __________.

Answer 6

Simple mechanism

Question 7

Catalysis of a reaction with two or more substrates usually involves what?

Answer 7

Transfer of groups from one substrate to the other

Question 8

Transfer of groups from one substrate to another can occur in what three ways?

Answer 8

1. Sequential with ternary complex
2. Non-Sequential with ternary complex
3. No ternary complex

Question 9

Define ‘ternary complex’

Answer 9

A protein complex that involves three molecules all associated with eachother. E.g. In this context, two substrates and an enzyme

Question 10

An example of a sequential mechanism includes?

Answer 10

Assimilation of pyruvate to lactate via lactate dehydrogenase

Question 11

An example of a non-sequential mechanism includes?

Answer 11

Formation of phosphocreatine from creatine via creatine kinase

Question 12

An example of a mechanism with no ternary complex includes?

Answer 12

Assimilation of aspartate to form glutamate via aspartate aminotransferase

Question 13

The aspartate to glutamate enzyme system is called what?

Answer 13

Double displacement (Ping Pong) reaction pathway

Question 14

Describe and explain the kinetics of an allosteric enzyme system.

Answer 14

One substrate binds to a subunit causing a conformational shift of the adjacent subunit, allowing the formation of an enzyme-substrate complex more easily and so on. This results in a sigmoidal curve when plotting reaction velocity against substrate concentration

Question 15

How can temperature effect enzyme activity?

Answer 15

Increased temperature leads to higher kinetic energy in the system, causing a higher frequency of successful collins (ES-complexes forming) - if temp. gets too high, it can denature the enzyme.

Question 16

How can pH effect enzyme activity?

Answer 16

pH changes the charge of amino acids and thus can interfere with the 3D structure of the enzyme - ultimately leading to its denaturation

Question 17

What three kinds of enzyme inhibitors are there?

Answer 17

1. Competitive
2. Non-competitive
3. Uncompetitive

Question 18

How do competitive inhibitors work?

Answer 18

They show an decreased Km and increased Vmax - the inhibitors bind more readily with the active site. Reducing the formation of enzyme-substrate complexes

Question 19

What does AZT stand for?

Answer 19

Azidothymidine

Question 20

What is AZT used to treat?

Answer 20

HIV

Question 21

Describe the mechanism of AZT in its combating HIV

Answer 21

1. Acts as a competitive inhibitor to reverse transcriptase

2. HIV cannot use this enzyme to produce its dsDNA from it ssRNA

Question 22

How do uncompetitive inhibitors work?

Answer 22

1. The inhibitors binds close to the active site of an enzyme
2. The enzyme-substrate complex forms
3. The ES complex is unable to dissociate - rendering the enzyme ineffectual

Question 23

How do non-competitive inhibitors work?

Answer 23

The inhibitor bonds non-covalently to a part of the enzyme - the shape of the active site changes and as a result, cannot interact with substrate molecules

Question 24

What happens to the Km of a system in the presence of a non-competitive inhibitor?

Answer 24

It doesn’t change as the substrate remains the same

Question 25

What is an example of a non-competitive inhibitor?

Answer 25

Cyanide

Question 26

CN- binds to Fe3+ cofactor of what enzyme?

Answer 26

Cytochrome c oxidase

Question 27

In effect, what does cyanide do to the body?

Answer 27

Stops the production of ATP

Question 28

What are the two main ways of regulating the activity of enzymes?

Answer 28

1. Use of allosteric enzymes

2. Use of covalently-modified enzymes

Question 29

An example of covalent modification in actions is _________.

Answer 29

Phosphorylation

Question 30

Allosteric factors are usually what?

Answer 30

Cell metabolites that bind non-covalently to a site on the enzyme

Question 31

How do these allosteric factors effect the enzyme?

Answer 31

They change its shape

Question 32

What two models explain the kinetics of an allosteric enzyme system?

Answer 32

1. Concerted model

2. Sequential model

Question 33

Explain the concerted model of allosteric enzyme kinetics.

Answer 33

1. Each subunit acts in the same way
2. Addition of one substrate/allosteric factor ‘locks’ all others in an open position
3. This makes binding of subsequent substrates much easier

Question 34

Explain the sequential model of allosteric enzyme kinetics .

Answer 34

1. The addition of a single allosteric factor/substrate causes a conformational shift in the adjacent subunit
2. This shift makes it easier for a substrate to bind to the adjacent substrate
3. This repeats for all subunits

Question 35

How does a feedback inhibition system work?

Answer 35

A build up of end product of a pathway, or key junction in a pathway, can ultimately slow the entire enzymatic system

Question 36

Enzymes can exist as inactive precursors called what?

Answer 36

Proprotein/proenzymes

Question 37

How are proenzymes activated?

Answer 37

They can be cleaved (process know as proteolytic cleavage) by proteases