Short Essay on Properties of Enzymes

Enzymes are biochemical catalysts. At this point it is necessary to refresh your mind as to what catalysts are.

Catalysts are chemical substances which in minute quantity accelerate the rates of chemical reactions without themselves being altered, destroyed or used-up during the course of the reaction and are recovered after the reaction is catalyzed.

Going by this definition, enzymes fall into the category of catalysts. However, enzymes (biological catalysts) are very efficient catalysts, often far superior to ordinary chemical (non-biological) catalysts. Enzymes differ from ordinary chemical catalysts in many other ways.

All these are because of their biological origin. The properties of enzymes and their differences or similarities with ordinary chemical catalysts are discussed below.

Catalytic Power

Most of the reactions in biological systems do not take place in perceptible rates in the absence of enzymes. Enzymes increase the rates of biochemical reactions by 10⁶ to 10¹² times of the uncatalyzed reactions. For example, one molecule of carbonic anhydrase can hydrate 10⁶ molecules of CO₂, per second.

This reaction is 10⁷ times faster than the uncatalyzed reaction.

Similarly, one molecule of catalase can decompose 10⁵ molecules of H₂0₂, per second; the enzyme catalase has iron in its active site which helps in the decomposition of H₂0₂. The efficiency of catalase in decomposing H₂0₂ can be realized by comparing¹ the decomposition of H₂0₂ with iron salts. One mg of iron in catalase is as effective as 10⁴ kg of inorganic iron in decomposing H₂0₂.

These examples indicate the great catalytic power of enzymes compared with non- biological catalysts.

Substrate Specificity

Enzymes are highly specific for the reactions they catalyze and for their choice of substrates than that of chemical catalysts.

They can discriminate between closely related substances, i.e., they have high degree of substrate specificity. A modified substrate may not be acted upon by the enzyme. For example, sucrose (also known as invertase) can hydrolyze sucrose but not sucrose phosphate.

Similarly, the enzyme urease can hydrolyze urea to form ammonia and carbon dioxide but cannot hydrolyze a slightly modified form of urea (e.g., chlorinated form of urea).

The above two enzymes show absolute specificity towards their respective substrates.

Some enzymes act on closely related substrates and catalyze reactions involving a particular chemical group. For example, the enzyme hexokinase transfer phosphate group from ATP to several different hexose sugars. Another example is the enzyme alcohols dehydrogenize which catalyze the oxidation of ethanol as well as other alcohols. Such enzymes exhibit group specificity.

A third group of enzymes exhibit stereo chemical specificity. These enzymes discriminate between mirror images of the same substrates. For example, L-amino acid oxidize catalyzes the oxidation of L-amino acids but does not act on its stereoisomer, D-amino acid.

Mild Reaction Conditions

Enzymes function in aqueous solutions under very mild conditions of temperature and pH. There are exceptions, however. Some protein-digesting enzymes operate in vacuoles with a pH near 4.0. And enzymes isolated from many thermophilic bacteria growing in hot springs can remain active at temperatures close to 100°C. Most of the enzymes, however, catalyze biochemical reactions under the conditions far milder than those required for most of the chemical reactions.

Enzymes, being protein in nature, are thermo labile. Most of the enzymes are denatured and lose their activity beyond 40-45°C. At high temperature, the enzymes are inactivated probably due to heat coagulation phenomenon.

Reversibility of Reaction

With very few exceptions, enzymes catalyze biochemical reactions which are reversible in nature. Depending upon the cellular requirement, the reactions are catalyzed either in the forward direction or in the backward direction.

This is not the case with the chemical catalysts. Enzymes like hexokinase and sucrose catalyze irreversible reactions. Carbonic anhydrase, alcohol dehydrogenase, succinate dehydrogenase and many other enzymes catalyze reversible reactions.

Capacity for Regulation

Enzymes regulate biochemical reactions to an extent not possible for chemical catalysts. Enzymes follow several strategies to regulate biochemical reactions and metabolic pathways. For example, enzymes can regulate metabolic pathways depending upon the requirement of the final product of the pathway by the cell.

If the final product of a metabolic pathway is not required by the cell, then this product may inhibit the first enzyme of the pathway, thereby preventing further formation of the product. This type of regulation is called feedback inhibition.