Metabolism 2 - Enzymes & Energetics

Question 1

What is an enzyme?

Answer 1

A protein that acts as a catalyst to induce chemical changes in other substances, itself remaining unchanged in the process

Question 2

What is Vin Gierke’s disease?

Answer 2

The most common glycogen storage disorder - an autosomal recessive deficiency of glycose 6-phosphatase. Causes hypoglycaemia, poor growth, distended abdomen and seizures.

Question 3

How do enzymes work?

Answer 3

They lower the energy barrier (activation energy) of a reaction by binding tightly to one or more substrate molecules via their active site. This puts strain on bonds present so they are easier to break.

Question 4

What are the first and second laws of thermodynamics?

Answer 4

First law- energy can neither be created or destroyed, it is simply converted from one to another.

Second law - in any isolated system the degree of disorder can only increase.

Question 5

What is entropy? How does it relate cellular reactions?

Answer 5

The amount of disorder in a particular system. Reactions always spontaneously proceed towards the products with greatest entropy. Biological systems are well ordered, as they taken energy from the surrounding environment and invest it in chemical reactions to maintain order.

Question 6

How does the increase in order of a cell affect the outside fo the cell?

Answer 6

As order increases inside the cell, disorder increases outside the cell as the molecules gain kinetic energy in the form of heat.

Question 7

What is free energy?

Answer 7

The amount of energy within a molecule that could perform useful work at a constant temperature. (G kJ/mol)

Question 8

When can a reaction occur spontaneously?

Answer 8

When deltaG is negative.

Question 9

How do coupled reactions work?

Answer 9

If a reaction in the cell is genetically unfavourable, it will take place when coupled with an energetically favourable one. If the overall deltaG is still negative, the reaction will continue. Most unfavourable reactions are paired with hydrolysis of ATP.

Question 10

What is the deltaG value of ATP hydrolysis?

Answer 10

-31kJ/mol

Question 11

What is the name of the bonds linking terminal phosphate groups in ATP?

Answer 11

Anhydride bonds.

Question 12

Why are enzymes needed if a reaction is genetically favourable?

Answer 12

Without an enzyme, the reaction will not occur at a rate that is useful for life. The deltaG value stays the same when an enzyme is present.

Question 13

Why does glucose not spontaneously combust when the deltaG is -2872kJ/mol?

Answer 13

An activation energy must be overcome - energy must intially be supplied in the form of heat to kick off the reaction.

Question 14

What is the transition state?

Answer 14

The particular conformation of the substrate in which the atoms of the molecule are rearranged both geometrically and electronically so that the reaction can proceed.

Question 15

How do enzymes make a reaction more likely to occur?

Answer 15

Enzymes put the substrates in a form where the bonds to be broken are stressed so the substrate resembles the transition state, making the reaction more likely to occur.

Question 16

Describe the barrier dam analysis of enzymes.

Answer 16

Balls represent the substrates, which are bounding up and down in energy level due to waves (which represent the thermal bombardment of substrate with the surrounding water molecules). When the barrier is lowered enough, the energetically favourable downhill movement of the ball can occur.

Question 17

How does the aqueous nature of the cellular environment affect reduction reactions?

Answer 17

When a molecule gains an elecron, it also simultaneously gains a proton.

Question 18

Describe the lock and key model.

Answer 18

The shape of the substrate matches the active site of the enzyme, which explains the specificity of enzymes for a specific substrate. Enzyme substrate complex forms, then products are released.

Question 19

Describe the induced fit model of enzymes.

Answer 19

The substrate induces changes in conformation of an enzyme, resulting in formation of the active site. When the products are released, the enzyme reverts back to the origional conformation.

Question 20

How does lysozyme work?

Answer 20

• Lysozyme hydrolyses alternating polysaccharide copolymers of NAM and NAG repeats present in bacterial cell walls. It contains two residues - Glu35 and Asp52.
• Glutamate protonates the oxygen in the glycosidic bond, becoming ionised. Water is deprotonated by the ionised Glu35. An OH group is added by the hydroxide ion of water.
• This is all stabilised by Asp52, which is ionised.

Question 21

What is the optimum pH for lysozyme and why?

Answer 21

pH 5.0 as Asp52 is ionised and Glu35 is unionised.

Question 22

How does temperature affect enzyme function?

Answer 22

Low temperature means low rate of reaction. High temperature means that the enzyme is denatured.

Question 23

How is NAD+ used in enzymes?

Answer 23

NAD+ cant function on its own - it helps enzymes in dehydrogenation reactions. It is called a co-enzyme. It accepts H+ and 2 electrons.

Question 24

Give an example of an enzyme where NAD+ is used.

Answer 24

It is used in the lactate dehydrogenase enzyme, first formed when pyruvate is converted to lactate. It then moves to the liver and is used by the same enzyme to regenerate pyruvate in the reverse reaction.