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Enzymes could be simple or conjugated (holoenzyme). Conjugated enzymes are formed of two parts – a protein part called apoenzyme and a non-protein part named co-factor. Co-factors are bound to the enzyme to make it catalytically active. There are three types of cofactors: prosthetic groups, co-enzymes, and metal ions. Prosthetic groups are organic compounds and are distinguished from other cofactors in which they are tightly bound to the apoenzyme. Co-enzymes are organic compounds but their association with the apoenzyme is only transient, occurring during the course of catalysis. A number of enzymes require metal ions for their activity which form coordination one or more coordination bonds with the substrate.

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Holoenzyme is the complete conjugate enzyme consisting of an apoenzyme and a cofactor. Cofactor may be organic or inorganic in nature. Organic cofactors are of two types-coenzyme and prosthetic group.

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A ribozyme is a ribonucleic acid (RNA) enzyme that catalyses a chemical reaction in a similar way to that of a protein enzyme. These are found in ribosomes and are also called catalytic RNAs.

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Competitive inhibition is a reversible inhibition where the inhibitor competes with the normal substrate for the active site of the enzyme. A competitive inhibitor is usually chemically similar to the normal substrate and therefore fits into the active site of an enzyme and binds with it. The inhibition is thus due to a substrate analogue. The enzyme now cannot act upon the substrate, and reaction products are not formed. E.g., the activity of succinate dehydrogenase is inhibited by malonate. Km value or Michaelis constant is defined as the substrate concentration at which half of the enzyme molecules are forming enzyme-substrate (ES) complex or concentration of the substrate when the velocity of the enzyme reaction is half the maximal possible. A smaller Km value indicates greater affinity of the enzyme for its substrate; hence, shows a quicker reaction. The competitive inhibitor decreases the affinity of the enzyme for the substrate, thus increases the Km value.

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Zinc is a cofactor (non-protein constituent) of the proteolytic enzyme carboxypeptidase. It makes the enzyme catalytically more active and their removal may result in loss of catalytic activity.

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The catalytic cycle of an enzyme action can be described as follows:

(i) First, the substrate binds to the active site of the enzyme, fitting into the active site. This leads to the formation of substrate enzyme complex. The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate.

(ii) The active site of the enzyme, now in close proximity of the substrate breaks the chemical bonds of the substrate and the new enzyme-product complex is formed.

(iii) The enzyme releases the products of the reaction and the free enzyme is ready to bind to another molecule of the substrate and run through the catalytic cycle once again.

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Inhibition of succinic dehydrogenase enzyme by malonate, which closely resembles the substrate succinate in structure, is a classical example of competitive inhibition. Such competitive inhibitors are often used in the control of bacterial pathogens.

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In competitive inhibition, the inhibitor closely resembles the real substrate in its molecular structure and inhibits the activity of the enzyme. Due to its close structural similarity with the substrate, the inhibitor competes with substrate for the substrate binding site of enzymes. Consequently, the substrate cannot bind and as a result, the enzyme action declines.

Inhibition of succinic dehydrogenase by malonate is one example of competitive inhibition. Malonate closely resembles the substrate succinate in structure. Such competitive inhibitors are often used to inhibit the growth of pathogenic bacteria.

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Enzymes catalysing a transfer of G group, (other than hydrogen) between a pair of substrates, S and S’ are known as transferases.

$\mathrm{S}–\mathrm{G}+{\mathrm{S}}^{\#}\to \mathrm{S}+{\mathrm{S}}^{\#}–\mathrm{G}$

•    Ligases catalyse the linking together of 2 compounds such as C – O, C – S, C – N bonds etc

•    Lyases catalyse removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds

Hydrolases are enzymes that catalyse hydrolysis of ester, ether, peptide , glycosidic, C – C, C – halide or P – N bonds.

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In peroxidase and catalase, which catalyze the breakdown of hydrogen peroxide to water and oxygen, haem is the prosthetic group and it is part of the active site of the enzymes.

Zinc is the cofactor in enzyme carbonic anhydrase.

RuBisCo is the most abundant protein in whole of the biosphere.

Succinate is the substrate of enzyme succinic dehydrogenase.

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There are number of cases in which non-protein constituents called co-factors are bound to the enzyme to make the enzyme catalytically active.

In these instances, the protein portion of the enzymes is called the apoenzyme.

Three kinds of co-factors are identified prosthetic groups, co-enzymes and metal ions. Prosthetic groups are organic compounds and they are tightly bound with apoenzyme. Co-enzymes are also organic compounds but their association with apoenzyme is only transient.