Enzymes: Properties and Mechanism of Enzyme Action

1. Proteinous nature:

Nearly all enzymes are proteins although some catalytically active RNA molecules have been identified.

2. Colloidal nature:

In the protoplasm, enzymes exist as hydrophilic colloids. Due to colloidal nature, they are isolated by dialysis.

3. Substrate specificity:

A given enzyme only catalyzes one reaction or a similar type of reaction. For example, maltase acts only on maltose while pancreatic lipase acts in a variety of fats. Sometimes, different enzymes may act on the same substrate to produce different end products. The substrate specificity of enzyme is based on amino acids sequence in the catalytic site as well as the optical isomeric form of the substrate.

4. Catalytic properties:

(i) Enzyme require in small concentration for any chemical change,

(ii) They don’t initiate the catalysis but accelerate the rate of catalysis by lowering the activation energy,

(iii) They remain unchanged at the end of reaction,

(iv) Their presence don’t alter the properties of end products,

(v) Enzymes accelerate the forward or reverse reactions to attain the equilibrium but don’t shift the equilibrium,

(vi) Usually enzyme catalyzed reactions are reversible, but not always,

(vii) They require hydration for activity.

5. Turn over Number (Enzyme efficiency):

It is the number of substrate molecules changed per unit of time per enzyme. Typical turn over number varies form 10² to 10³ sec^-1. For example the turn over number for sucrase is 10⁴, that means, one sucrase molecule convert 10,000 sucrose into products. Similarly, it is 36 million for carbonic anhydrase (fastest enzyme) and 5 million for catalase (2^nd fastest enzymes). Enzyme efficiency is very low in Lysozyme.

6. Sensitivity:

Enzymes are highly sensitive to change in pH, temperature and inhibitors. Enzymes work best at a narrow range of condition called optimum.

(i) Temp:

The optimum temp of enzymes is 20-35°C. They become inactivated at very low temperature and denatured (destroyed) at very high temp i.e. greater than 45°C. Low molecular weight enzymes are comparatively more heat stable. In archebacterium Pyrococcus furious, the optimum temperature of hydrogenise is greater than 95°C. This heat-stable enzyme enables Pyrococcus to grow at 100°C.

(ii) pH:

The optimum pH of most endoenzyme is pH 7.0 (neutral pH). However, digestive enzymes can function at different pH. For example, salivary amylase act best at pH 6.8, pepsin act best at pH2 etc. Any fluctuation in pH from the optimum causes ionization of R-groups of amino acids which decrease the enzyme activity. Sometime a change in pH causes the reverse reaction, e.g. at pH 7.0 phosphorylase break down starch into glucose 1- phosphate while at pH5 the reverse reaction occurs.

(iii) Inhibitors:

Enzymes are also sensitive to inhibitors. Inhibitors are any molecules like cellular metabolites, drugs or toxins which reduce or stop enzyme activity. Enzyme inhibitors are of 2 types i.e. reversible and irreversible.

Irreversible Inhibitors (=Inactivators) Covalently bind to amino acid residue of catalytic site, and permanently inactivate the enzyme, e.g., penicillin covalently attach to a serine residue in the active site of the glycopeptide transpeptidase enzyme that forms cross-linking in bacterial cell wall. Phenylmethylsulfonyl fluoride forms a covalent bond with the catalytic site serine residue of proteases like trypsin, chymotrypsin etc. and thus used during enzyme isolation.

A competitive inhibitor competes with a substrate to bind reversibly to catalytic site. The action of such type of inhibitor is overcome by increasing substrate concentration, dilution or dialysis. For example, maloiatc is a competitive inhibitor which competes with succinate and inhibits activity of succinate dehydrogenase. Similarly sulpha dings compete with PABA (Para-amino benzoic acid, a substrate) which inhibits the synthesis of folic acid (a vitamin) in bacteria.

An uncompetitive inhibitor binds reversibly to the enzyme substrate complex, but not the free enzyme.

A non-competitive inhibitor or mixed inhibitor binds to both free enzyme and the enzyme- substrate complex.

An allosteric or feedback inhibitor is the end product of a metabolic pathway that inhibits the activity of the first enzyme of that pathway.

Mechanism of Enzyme Action:

Enzymes neither initiate the reaction nor affect the equilibrium ratio of reactants and products. Rather, enzymes accelerate the rate of reaction 10⁸ to 10¹² times in both directions to attain the equilibrium position.

Activation Energy:

The kinetic or collision theory states that for molecules to react they must collide and must possess sufficient energy to overcome the energy barrier for reaction. The minimum amount of free energy required to overcome the energy barrier, so that substrates transform into the transitional state, is called activation energy or free energy of activation (∆G). The transition state is an unstable complex develops at some point in the reaction between the substrate(s) and the products (P), and has the highest free energy in the reaction pathway. Thus,

Activation energy increases kinetic energy of substrates and brings about the forceful collisions between Enzyme (E) and substrates (S). In a non-enzymatic reaction, the ∆G^≠ is very high which can’t be provided by any living system. For example, ∆G^≠ is 32 Kcal/mole for acid hydroslysis of sucrose. But enzymatic hydrolysis of sucrose requires only 9.4 kcal/ mole free energy.

Therefore, it is clear that Enzyme lowers the free energy of activation (∆G^≠) for a biochemical reaction by more than 50% due to following reasons:

(i) Raising the ∆G of the substrate,

(ii) Stabilizes the transition state,

(iii) Release of binding energy (kinetic energy) when substrates establish bonds with catalytic site,

(iv) Weakening the bonds of substrate by nucelophilic and electrophilic attack by catalytic site.

Free energy change:

The difference in energy level between the substrate and product is called the change in Gibbs free energy (G). The ∆G indicates whether a reaction is thermodynamically favourable or not. The negative value of ∆G indicates that the reaction is energetically favourable. If the ∆G is positive it indicates that the reaction is not energetically favourable and endergonic (require input of energy). The ∆G of a reaction is independent of the rate of a reaction while ∆G^≠ governs the rate of reaction. Therefore, ∆G is different from ∆G^≠.