Enzymes as biological catalysts

Question 1

What do enzymes do and how does this relate to the position of equilibrium?

Answer 1

Enzymes catalyse many chemical reactions which together make up the process of metabolism. They speed up the rate of a reaction and so speed up the rate at which a reaction reaches equilibrium however they DO NOT alter the position of equilibrium

Question 2

Complete the phrase:

Enzymes can lower the activation energy barrier by…

Answer 2

Providing an alternative reaction pathway

Question 3

True or false: Enzymes are ALWAYS proteins

Answer 3

False - enzymes are USUALLY proteins however, there are some exceptions such as some types of RNA (e.g ribozymes)

Question 4

Name 3 characteristics of enzymes?

Answer 4

Enzymes are:

1. Efficient - they work at body temp, in aqueous solution and near neutral pH
2. Specific - Each enzyme has a limited range of substrates
3. Potent - Each enzyme molecule can convert many substrates into product per second

Question 5

Is the transition state intermediate at the top of the energy barrier stable or unstable?

Answer 5

Unstable - The transition state is the intermediate that has the greatest free energy

Question 6

Name 2 factors that can effect enzyme activity?

Answer 6

1. Temperature

2. pH

Question 7

What is the difference between co-factors and co-enzymes?

Answer 7

Co-factors - These are metal ions (inorganic) - e.g zinc, iron, copper. They form a metal co-ordination centre in enzyme often called a metalloprotein

Co-enzymes - These are organic molecules, many are derived from vitamins. They mostly associate with enzyme only transiently (for short periods of time) and they can change charge or structure during the course of a reaction, but are regenerated

Question 8

What is the term given to a tightly bound coenzyme?

Answer 8

This is a prosthetic group (e.g haem in haemolglobin)

Question 9

What is an enzyme WITHOUT a cofactor called?

Answer 9

Apoenzyme

Question 10

What is an enzyme WITH a cofactor called?

Answer 10

Haloenzyme

Question 11

Therfore, a cofactor + apoenzyme = what?

Answer 11

cofactor + apoenzyme = haloenzyme

Question 12

Where on an enzyme does a substrate bind?

Answer 12

Substrate binds to the active site

Question 13

What is the difference between the lock and key model compared to the induced fi model?

Answer 13

Lock and key model - enzyme and substrate are complementary to each other (have exactly correct shapes)
Induced fit model - Conformation change of enzyme’s active site upon binding of the substrate to make it fit better

Question 14

What are the active sites of the three pancreatic serine proteases enzymes like?

Answer 14

1. Chymotrysin - hydrophobic pocket binds aromatic amino acids
2. Trypsin - negatively charged aspartic acid interacts with the positively charged lys or arg
3. Elastase - active site partially blocked, only amino acids with small or no side chains can bind

Question 15

What are isozymes?

Answer 15

Isozymes are isoforms of enzymes, they catalyse the same reaction but have different properties and structure (and sequence)

Question 16

Give an example of an isozyme?

Answer 16

Lactate dehydrogenase is an example that has 5 isozymes from LDH 1 to LDH5. It has 2 subunits that make up each LDH gene:

1. H (Heart) - promotes aerobic metabolism
2. M (Muscle) - promotes anaerobic metabolism

Question 17

What are zymogens?

Answer 17

Zymogens are inactive precursors of an enzyme. They are irreversibly transformed into active enzymes by cleavage of a covalent bond

Question 18

What can regulation of enzyme activity by phosphorylation result in and what enzyme carries out phosphorylation?

Answer 18

Regulation of enzyme activity by phosphorylation can convert enzyme to active or inactive form. Phosphorylation reactions are carried out by protein kinases

Question 19

True or False: Relative amounts of isozymes in tissues are useful for diagnostic purposes

Answer 19

True