Enzyme are partly or entirely a protein that can tremendously increase the efficiency of a biochemical reaction and is generally specific for that reaction.
The term enzyme (Gr. en = in + zyme = living) was first discovered by W. Kuhne in 1878 while working on fermentation.
Nature of Enzymes – Physical Properties
The enzymes are proteins wholly or partly. They increase efficiency of a biochemical reaction by increasing their speed, therefore these are also called biological catalysts. The enzymes are vital because in their absence reactions in the cell would be too slow to sustain life. They exhibit many of the physical properties of proteins and inorganic catalysts and thus can be characterized in the followings:
- Enzymes are molecules of higher molecular weight. Peroxidase, one of the smallest enzyme, has molecular weight of 40;000, whereas the catalase, one of the largest enzyme has higher molecular weight of 290,000 approx.
- Enzymes are colloidal substances. The enzymes being proteins are colloidal in nature, therefore form colloidal suspension in the cytosol. They form micelle and provide large surface area of catalytic actions.
- Enzymes are active in small amounts, i.e., only a small amount of enzyme is necessary to convert a large amount of substrate to product in a biochemical reaction. The number of moles of substrate converted per minute by 1 mole of enzyme is called turnover number of the enzyme. This number may vary from 100 to over 3,000,000.
- Enzymes are unchanged at the end of a reaction.
- Enzymes speed up the rate at which equilibrium position is reached. A catalyst speeds up the rates of both the forward and backward reactions, and the rate at which equilibrium position is attained. A catalyst does not alter the position of equilibrium. Similarly, an enzyme hastens the completion of a reaction, it will not affect the equilibrium of that reaction.
- Enzymes lower the reading molecules.
- Enzymes are extremely specific. Most enzymes are specific to one particular type of substrate molecule, and usually one isomer of that substance. Other enzymes are specific to a group of similar substances, or to a particular type of chemical linkage.
- Enzymes can be denatured. Like most proteins, the enzymes can be denatured by heat and lose their enzymatic activity. Other factors such as metal ions such as those of lead, mercury and silver; concentrated acids and bases; and ultra violet light also being denaturation. Many organic solvents also denature the enzymes.
Chemical Nature of Enzymes – Chemical Properties
Every known enzyme is protein wholly or has a protein as major parts of its structure. The non-protein part of enzyme is called cofactor. The enzyme cofactor complex is called holoenzymes. Whereas enzyme portion without cofactor is known as apoenzyme.
Apoenzyme – Protein Part of Enzymes
Protein of enzymes consists of one or more polypeptide chains of tens to hundreds of amino acids. The composition and size of each protein depends upon the kind and number of its amino acid sub-units.
The total number of amino acid sub-units varies greatly in different proteins and so protein molecular weight also varies from 11,500 (ferredoxin) to 500,000 (ribnlose bisphosphate carboxylase) grams mole-1. The chains of complex enzymes are held together by non-covalent bonds. Often ionic or hydrogen bonds and can be separated in vitro.
Many enzymes that do not have prosthetic group require another organic compound for their activity. These compounds are called coenzymes. These are usually not tightly held to the enzymes. The common examples of coenzymes are NAD (nicotinamide adenine dinucleotide), NADP (nicotinamide adenine dinucleotide phosphate). Coenzyme A, and ATP. Vitamins produced by the plants are source of coenzymes, e.g., NAD is derived from vitamin nicotinic acid.
Enzyme require inorganic ions for their efficient enzymatic activity. These are thought to mold either the enzyme or the substrate into a shape such that an enzyme substrate complex can be formed, hence in the presence of calcium ions. The other activators are metal ions such as Zn, Mg, Ca, Co, Fe etc.