•Enzymes control cell metabolism by regulating how and when reactions occur. Using this very simple pathway as an example:

A → B → C → D •The final product is substance D, the

chemical needed by the living organism. The pathway needs three different enzymes and when D is no longer needed or if too much has been produced, one of the three enzymes is ‘switched off’.

The lock and key hypothesis

•The induced fit hypothesis

Al-hadad D. Kalon | Prof. Joel G. Matamis | Bachelor in Medical Laboratory Science | SHSP | St. Dominic College of Asia

What are Enzymes?

• ENZYMES are complex chemicals that control reactions in living cells. They are biochemical catalysts speeding up reactions that would otherwise happen too slowly.

• The chemical which an enzyme works on is called its substrate.

• An enzyme combines with its substrate to form a short-lived enzyme/substrate complex. Once a reaction has occurred, the complex breaks up into products and enzyme.

The role of enzymes in an organism

The Chemical nature of enzymes

ENZYMES

The specificity of enzymes

•Enzymes are specific: each enzyme usually catalyses only one reaction.

• Enzymes combine with their substrates to form temporary enzyme-substrate complex.

• Enzymes are not altered or used up by the reactions they catalyze, so can be

used again and again.• Enzymes are sensitive to temperature and pH.

• Many enzymes need cofactors in order to function.

• Enzyme function may be slowed down or stopped by inhibitors.

The factors that affect enzyme activity also affect the functions of the cell and ultimately the organism. Enzymes are proteins and their function is therefore affected by:

Temperature

pH

Substrate concentration

Enzyme concentration

Cofactors

Inhibitors

Factors affecting enzyme activity

Different EnzymesEnzyme are classified according to the type of reaction they catalyse into six groups:1. Oxi-reductases – this are enzymes that catalyse oxidation-

reduction reactions. Oxido-reductases are further classified into five subgroups:

A. Oxidases –these are enzymes that catalyse direct transfer of hydrogen to oxygen and form water.

B. Aerobic Dehydrogenases –these enzymes that catalyse direct transfer of hydrogen to oxygen and form hydrogen peroxide.

C. Anaerobic Dehydrogenases- these enzymes cannot transfer hydrogen directly to oxygen but hydrogen is indirectly transferred to oxygen through many hydrogen carriers.

D. Hydroperoxidase- these enzymes catalyse direct incorporation of oxygen into substrate. e.g.

i. Dioxygenases (true oxygenases): these enzymes catalyse incorporation of two oxygen atoms into substrate.

ii. Monooxygenases: these enzymes incorporate one oxygen atom into substrate.

2. Transferases- these are that catalyze transfer of a chemical group from one compound to another. They include:

A. Transaminases: these are enzymes that catalyze transfer of amino group from amino acid to a-keto acid producing new amino acid and new keto acid e.g.

i. Alanine transaminase (ALT): it is also called serum glutamic pyruvic transaminase (SGPT).

ii. Aspartate transaminase (AST): it is also called serum glutamic oxaloacetic transaminase (SGOT).

B. Acyl transferases- these enzymes catalyse the transfer of acyl group to compounds.

C. Methyl tranferases- these enzymes transfer methyl group from methyl donor usually active methionine to the substrate e.g. formation of epinephrine from norepinephrine.

D. Phospotransferases- these enzymes catalyze transfer of phosphate group e.g. hexokinase and glucokinase both catalyse transfer of phosphate group from ATP to glucose.

3. Hydrolases- these enzymes catalyse cleavage of their substrate by addition of water. All the digestive enzymes are hydrolases.

A. Enzymes hydrolyzing glycosidic link in carbohydrates- these enzymes catalyse hydrolysis of carbohydrates e.g. maltase, lactase, sucrose and amylase.

B. Lipase: enzyme that hydrolzes triglyceride into glycerol and free fatty acids.

C. Proteases: enzymes that catalyse hydrolysis of proteins e.g. pepsin and trypsin.

D. Phosphatases: enzyme that catalyse hydrolysis of phosphoric acid esters e.g.

i. Phosphomonoesterases that catalyse hydrolysis of phosphoric acid monoesters as glucose-6-phosphatase.

ii. Phosphodiesterases that catalyse hydrolysis of phosphoric acid diesters e.g. the enzyme that catalyzes the hydrolysis of cyclic adenosine monophosphate to AMP.

4. Lyases (desmolases)- these enzymes catalyse cleavage of substrates or removal of chemical groups by mechanisms other than addition of water i.e. by mechanisms other than hydrolysis. They include:

A. Aldolase: it is an enzyme that splits aldehyde from alcohol e.g. fructose-1-6-diphosphate aldolase.

B. Dehydratases: these enzyme catalyze removal of water from their substrates e.g. funarase and carbonic anhydrase.

C. Decarboxylases: these enzymes catalyze the removal of CO2 from their substrate. They need pyroxidal phosphate as coenzyme.

D. Phosphorylases: these enzymes catalyse cleavage of their substrates by addition of phosphoric acid e.g. glycogen phosphorylase.

5. Isomerases- these enzymes catalyse intramolecular rearrangement, so they catalyse conversions between optical, positional and geometric isomers. They include:

A. Aldose-ketose isomerases: these enzymes catalyse interconversion between aldoses and ketoses e.g.

B. Epimerases: these enzymes catalyse interconversion between epimers e.g.

C. Mutases: these enzymmes catalyse transfer of chemical group from one position to another in the same compound e.g.

D. Racenases: these enzymes catalyse interconversion between D and L enantiomers e.g.

E. Cis-Trans isomerases: these enzymes catalyse interconversion between cis and trans geometric isomers e.g.

6. Ligases- these enzymes catalyse the binding of twi molecules to form one molecule. They need energy which is derived from ATP e.g. glutamine synthetase.

CLINICAL SIGNIFICANCE OF PLASMA ENZYME CONCENTRATIONS

Enzymes are globular proteins. They have a complex tertiary and quaternary structure in which polypeptides are folded around each other to form a roughly spherical or globular shape. The overall 3D shape of an enzyme molecule is very important: if it is altered, the enzyme cannot bind to its substrate and so cannot function. Enzyme shape is maintained by hydrogen bonds and ionic forces.