General characteristics:
- Biological catalysts
- Neither consumed nor produced
- Do not cause reactions to take place
- Do not alter the equilibrium
- Invariably proteins
- Highly specific
- Function within a moderate pH and temperature

- Native conformation of the protein required for activity (if enzyme is simple protein).
- And for conjugated also on cofactors availability.

Cofactor

- Inorganic like metals

- Organic e.g NAD+, NADP+ (Co-enzymes)

Metals

- Loosely bound-activated enzyme

- Tightly bound-metallo-enzymes

- Cofactor if tightly bound – prosthetic group e.g. FAD.

- Unit of enzyme activity – Transformation of 1 mol of substrate/ min at 25C under optimal conditions.

- Specific activity – Number of units of enzyme activity/ milligram of enzyme protein.

- Katal (kat) – amount of enzyme activity that transforms 1 mol of substrate/ sec.

Classification:

1 – Oxidoreductase – are involved in oxidation and reduction e.g. Lactate dehydrogenase.

2 – Transferases – catalyze transfer of C, N or P containing groups e.g. Serine hydroxymethyl transferase.

3 – Hydrolases – catalyze cleavage of bonds by addition of water e.g. Urease.

4 – Lyases – catalyze cleavage of C-C, C-S and certain C-N bonds e.g. Pyruvate decarboxylase.

5 – Isomerases – catalyze racemization of optical or geometric isomers e.g. Methylmalonyl CoA mutase.

6 – Ligases – joins two molecules together at the expense of ATP e.g. Pyruvate carboxylase

Cytosol:
Glycolysis
HMP pathway
Fatty acid synthesis

Mitochondria:
TCA cycle
Fatty acid oxidation
Decarboxylation of pyruvate

Nucleus:
DNA and RNA synthesis

Lysosome:
Degradation of complex macromolecules

Multienzyme system:

1) Least Organized: e.g. glycolysis

2) Organized: e.g. Pyruvate dehydrogenase complex

3) Highly organized: e.g. Electron transport chain

4) Compartmentalized: e.g. Fatty acid synthesis and oxidation

Mechanism of enzyme action:

- Michaelis and Menten proposed

Enzyme Inhibition

Competitive inhibition

- Between substrate an inhibitor for binding to the active site

Non competitive inhibition (Irreversible)
e.g. organophosphorus compounds bind with cholinesterase

Amount of Enzyme –
- Enzyme turnover
- Induction and repression of enzyme synthesis
- Adaptation e.g. liver enzyme during dietary carbohydrate
- Induction e.g. by barbiturates

Feed-forward –
- Linking the two kinase activities e.g. Phosphofructokinase acitivy activates pyruvate kinase.

Product inhibition -

Acceptor control -
e.g. 1, 3- diphospoglycerate + ADP

3 - Phosphoglycerate + ATP

Covalent modification -
e.g. phosphorylation/ dephosphorylation.

Regulatory or Allosteric enzymes -
= allosteric means ‘other site’

Non-functional plasma enzymes
- no physiological function in blood
- increased level suggest destruction

• Exocrine enzymes – diffuse into plasma
e.g. pancreatic amylase, lipase, bile ALP,ACP

• True intracellular enzymes
- normally absent from circulation
e.g. lactate dehydrogenase, aminotransferases

Entry of enzymes into the blood
• Rate of enzymes leak from cells
• Altered rate of enzyme production
- increased synthesis of particular enzyme
- proliferation of particular type of enzyme producing cells