E N Z Y M E S

Enzyme:

– mostly proteins, but some catalytic RNA molecules

(ribosymes)

– extraordinary catalytic power

– high degree of specificity for their substrate

– tremendously accelerate chemical reactions

– function in aqueous solutions under very mild

conditions of temperature and pH

– can be regulated

History of enzyme research:

– late 1700s biological catalysis recognized and described– early 1800s conversion of starch to sugar by saliva– 1850 Louis Pasteur fermentation of sugar into alcohol by

yeast is catalysed by “ferments”– 1897 Eduard Buchner yeast extractum- can work without

living cell- later Frederick Kühne called these moleculesENZYMES

– 1926 James Sumner- ureasehe postulated that “all enzymes are proteins”

– 1930s John Northrop and Moses Kunitz crystallized trypsin, pepsin and other digestive enzymes

– JBS Haldane’s treatise entitled “Enzymes”– Since the later part of the twentieth century intensive research on

enzymes- purification of thousands of enzymes and elucidation of the structure and chemical mechanism of them

E. C. (Enzyme Commission) number

enzymes are classified by the reactions they catalyze

EC 1. Oxidoreductases

These enzymes catalyze oxidation and reduction reactions.

- dehydrogenases act on the following functional groups as e- donors: -CH2=CH2-, -CH2-NH2-, -CH=NH, NADH and NADPH

- oxidases 2 e- transfer to O2- usually H2O2 formation

- oxygenases catalyze incorporation of both atoms of O2 into the substrate

- hydroxylases incorporation of one atom of O2 (steroid hydroxylases)

- peroxidases utilize H2O2 rather than O2 as the oxidant

- catalase unique in that, H2O2 serves as both donor and acceptor. Function in the cell to detoxify H2O2

H2O2 + H2O2 2 H2O + O2

EC 2. Transferases

They are involved in transferring functional groups between donors and acceptors- transferred amino, acyl, phosphate one-carbon and glycosyl groups.

- aminotransferases- transfer amino group to a keto acid acceptor

- kinases- transfer phosphoryl group to alcohol or amino group acceptors

EC 3. Hydrolases

They catalyze hydrolysis of different bonds. The donor group is transferred to water.

The cleavage of the peptide bond is a good example of this reaction.

EC 4. Lyases

They add or remove the elements of H2O, NH3 or CO2.

- decarboxylases

- aldolases

-hydratases

- dehydratases

-synthases

EC 5. Isomerases

They catalyze isomerization of several types.

- racemases, epimerases- inversion at asymmetric carbons

- isomerases (cis-trans)

- intramolecular transferases

- mutases- intramolecular transfer of a group

EC 6. Ligases

These enzymes are involved in synthetic reactions where two molecules are joined.

- synthetases

- carboxylases

– proteins or ribosymes– integrity the native protein conformation– molecular weight 12000- 1 million D– require no chemical group for activity– require COFACTOR – require COENZYME– PROSTHETIC group

Holoenzyme: complete, catalytically active enzyme together with its bound coenzyme and/or metal ions.

Apoenzyme (apoprotein): protein part of such an enzyme

– catalytic power

– specificity for their substrate

– accelerate chemical reactions

– transform different energy forms

– can be regulated

Generally about enzymes

Enzymes influence the reaction rate, but they do not

affect reaction equilibria!

Enzymes enhance reaction rate by:

stabilizing the transition state

lowering activation energies

– entropy reduction (restriction in the relative motions of the substrates that are to react)

– desolvation of the substrate

– change in conformation of the substrate

– exact fit of enzyme and substarte

Binding energy

– catalytic group (break or form different binds)

– small size (enzyme itself is large)

– three dimensional structure

– binding substrate with multiple weak interactions – this gives specificity

Active center of enzymes

Specific catalytic groups contribute to catalysis

Acid-base catalysis

specific

general

Covalent catalysis

Metal ion catalysis

Enzyme kinetics

Simple enzymatic reaction:

E + S ES EP E + P

v0 = vmax [S]KM + [S]

Enzyme inhibition

– Methanol or ethylene-glycol poisoningTh: ethanol infusion

– sulfa drugs (sulfanilamide) - antibiotics - competes with p-aminobenzoic acid, which is required for bacterial growth

– Methotrexate - biosynthesis of DNA and RNA requires folic acid, and its structural analog- chemotherapic agent

Medical significance of competitive inhibition:

– enzymes with different molecular structure, catalyzing the same

biochemical reactions

– they are either the products of different genes or complexes of a

few subunit types, mixed at different ratios

– heterogenity in their KM value (- different substrate affinity)

– represented in different tissue, cell types or compartments by

their different isoforms

Izoenzymes:

– ways can be distinguished:

~ electrophoretic separation

~ application of isoform specific antibodies

~ application of isoform specific inhibitors

~ on the basis of different heat sensitivity

CPK1 BB CPK2 MB myocardium CPK3 MM skeletal muscle