Enzymes 2

Question 1

What reaction do Glucokinase & hexokinase catalyze?

Answer 1

Glucose +ATP -> G-6-P + ADP

This traps the glucose in the cell as G-6-P

Question 2

What are the kinetic properties of Gluco-&Hexokinase

Answer 2

Glucokinase = High Km & Vmax
Hexokinase = Low Km & Vmax

Question 3

Where are glucokinase & hexokinase found?

Answer 3

Glucokinase is in the liver

Hexokinase is in the muscles & other tissues

Question 4

What is the purpose of the Glucokinase/hexokinase pair?

Answer 4

The hexokinase has the higher affinity so even at low glucose concs. the tissues will still have dibs on the glucose.
However when glucose conc. exceeds the hexokinase Vmax the glucokinase will step in & trap the extra in the liver

Question 5

What does finding lots of intracellular enzymes in the plasma tell us?

Answer 5

There been cell damage due to disease or trauma

Question 6

Define Isoenzymes:

Answer 6

Enzymes that catalyse the same reaction

Question 7

How to we seperate & study plasma proteins?

Answer 7

Electrophoresis

Question 8

In what two ways are enzymes used in diagnosis?

Answer 8

• comparing activity to the normal

- Examine enzymes by electrophoresis

Question 9

In what two ways can an enymatic reaction occur when theres two substrates?

Answer 9

With or withut a ternary complex

Question 10

Define an allosteric enzyme?

Answer 10

Allosteric enzymes are muli-subunit & contain many active sites

Question 11

What is cooperative binding?

Answer 11

One substrate binding to a subunit causes changes in the other subunits that make further binding easie

Question 12

Best example of a cooperative binding molecule?

Answer 12

Haemoglobin

Question 13

What are the 2 types of enzyme inhibtion?

Answer 13

Competetive & non-competetive

Question 14

How do competetive inhibitors work?

Answer 14

They resemble the substrate & bind to the active site.
This reduces the enzymes affinity for the substrate
Therefore Km increases

Question 15

Why is Vmax unchanged when using a competetive inhibitor?

Answer 15

Becuase diluting the inhibitor with more substrate (increasing [S]) can overcome competetive inhibition

Question 16

What effect does a competetive inhibitor have on an enzymes lineweaver-burke plot?

Answer 16

Increases the gradient

Question 17

Why make inhibitors that mimic the transistion state over the substrate?

Answer 17

Enzymes have a maximum interaction with the transistion state (higher affinit bascially)

Question 18

Why is it hard to produce transition state analogues?

Answer 18

They are difficult to isolate as theyre so temporary

Question 19

How do non-competetive inhibitors wokr?

Answer 19

Bind to the enzyme but not the active site
Therefore Km is unchanged
However Vmax is decreased

Question 20

What effect do non-competetive inhibitors have on lineweaver-burke plots?

Answer 20

Non-compettive inhibitors result in an increased Y-intercept (1/Vmax) and an increased gradient (Km/Vmax)

Question 21

What are the 4 ways enzyme pathways are regulated?

Answer 21

Allosteric regulatory enzymes
Covalent modification
Feedback inhibition
Proteolytic Cleavage

Question 22

Explain feedback inhibitio?

Answer 22

Build up of an end-product or key-junction slows the whole pathway by inhibiting an earlier enzyme

Question 23

What is an allosteric effector?

Answer 23

A metabolite that binds to the enzyme altering its structure.

Question 24

Are allosteric effectors inhibitory?

Answer 24

They can be both inhibitory or activators

Question 25

What type of inhibitor are allostetic effectors?

Answer 25

Non-competetive

Question 26

What are the two models of allosteric enzyme kinetics?

Answer 26

The concerted model

The sequential model

Question 27

Explain the concerted model?

Answer 27

• All subunits flip between binding & non-binding conformations simultaneously
• One binding to a substrate locks them all in the ‘open’ position
• In this way low [S] has sensitised the enzyme to more S.

Question 28

What shape does an allosteric enzyme M-M graph take?

Answer 28

A sigmoidal curve

Question 29

What does the allosteric enzymes Sigmoidal curve suggesT?

Answer 29

Low [S] sensitizes the enzyme to better bind higher concs of [S]

Question 30

How do allosteric effectors affect the concerted model?

Answer 30

Allosteric activators would lock a concerted model allosteric enzyme in the open position
Reverse for allosteric inhibitors

Question 31

Explain the sequential model:

Answer 31

The subunits dont flip between ope/closed

• Open starts open
• Substrate binds to it
• Causes conformational change in next subunit (opening it)

Question 32

What is the most common method of regulatory covalent modification?

Answer 32

Phosphorylation

Question 33

What enzymes phosphorylate others?

Answer 33

Kinases add phosphoryl groups

Phosphotases remove them

Question 34

How does phosphorylating an enzyme regulate it?

Answer 34

Most enzymes have multiple phosphorylation site

• Phosphorylating more sites inactivates the enzyme in increments
• This allows fine tuning of the enzymes activity

Question 35

Whats a proprotein/proenzyme?

Answer 35

An inactive precursor to an enzyme

Question 36

What enzymes conduct proteolytic cleavage?

Answer 36

Proteases

Question 37

What is proteolytic cleavage?

Answer 37

Cleavage of a proenzyme to release the active enzyme

Question 38

Examples of proteolytic cleavage regulation?

Answer 38

• Digestive enzymes

- Insulin