Knowledge of catalytic effect of inorganic elements and compounds led to the recognition of enzymes as biocatalysts. Enzymes were soon recognized as proteins.
The enzymes made up of exclusively of proteins are known as simple protein enzymes and those with additional non-protein parts are known as conjugated protein enzymes. The latter enzymes are functional only when the protein (apoenzyme) and the non-protein part (prosthetic group, co-enzyme or metal cofactor) remain together forming the complete enzyme (holoenzyme).
Prosthetic group is tightly bound to enzyme, which may be a complex organic molecule (e.g., vitamins or iron porphyrins) or metal ions (single metal prosthetic group). The other non-protein part rather loosely bound (separable by dialysis) may be a complex organic molecule (co-enzyme) or a metal ion (metal cofactor or metal activator). Thus, the enzyme has been defined as “simple or conjugated protein acting as specific catalyst”.
The main characteristic features of enzymes are: (i) catalytic properties, (ii) specificity of enzyme action, (iii) reversibility of action, (iv) heat stability (thermolabile nature) and (v) stability.
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Enzymes are named according to the type of reaction they catalyse or the type of substrate they utilise with a suffix ‘-ase’. The IUB (1961) classified the enzymes into six major groups-(i) oxido-reductases, (ii) transferases, (iii) hydrolases, (iv) isomerases, (v) lyases and (vi) synthetases.
A very small portion, constituted of 3-12 amino acids of the enzyme, called the active site comes in direct contact with the substrate. Such a contact enables the enzyme and substrate to fit together properly and form enzyme substrate complex (ES). In doing so, the enzymes (with surface for adsorption) bring the substrate or reactant molecules in juxtaposition and also lower the activation energy required by them to be able to react and form product(s). In light of all these, we can define that enzymes are proteins that act as biological catalysts to speed up the rates at which chemical reactions occur by lowering the activation energy.
Two models have been suggested for formation of enzyme-substrate complex that are held together by weak interactions or even by short lived full covalent bonds. The ‘Lock and Key’ model suggests a specific fit between enzyme and substrate at the active site, like the appropriate lock and key arrangement. Whereas the ‘induced fit’ model suggests that the active site being not rigid adopts to a form in which the substrate can fit properly (like the thin hand gloves). Such a shape adoption and the weak bonds between the enzyme and the substrate imposes enough strains to encourage some specific reactions.
Product(s) form by removal/addition of hydrogen (oxido-reductases), group transfer (transferases), hydrolytic cleavage (hydrolases), intramolecular rearrangement (isomerases), reversible bond cleavage (lyases) and union between molecules (synthetases) of the substrates or reactants.
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The precision fit between enzyme and substrate molecules is crucial to the catalytic process.
Enzyme activity is affected by a number of factors, in the medium (the cell), viz., (i) substrate concentration, (ii) enzyme concentration, (iii) optimum pH and (iv) temperature.
A variety of compoundeds, natural or synthetic, that mayor may not be structural analogues can bind reversibly or irreversibly to the specific enzymes and alter their activity. In competitive inhibition the inhibitor competes with the substrate for the active site. This can be reversed if concentration of the. substrate is increased. In non-competitive inhibition the inhibitor binds to the ‘inhibitor binding site’ that is not identical to the active site.
These inhibitors do not compete with the substrate but somehow decrease the velocity of catalytic reaction which cannot be overcome by increasing the substrate concentration. The allosteric effectors act at a site called ‘allosteric site’ which lies at a distance from the active site. It modifies the shape· of active site so that it either cannot accommodate its substrate (in case of allosteric inhibitors) or can bind efficiently (in case of allosteric activators). In feedback inhibition the end product of a metabolic pathway may exert a negative effect on the initial step of the reaction chain.