Enzymes are the most important group of proteins which are biologically active. They tremendously increase the efficiency of a biochemical reaction and are specific for each type of reaction. Without these enzymes the reaction would proceed at a very slow speed making life impossible.
Enzymes are composed of hundreds of amino acids joined together and coiled upon themselves to form a globular structure. The catalytic activity is restricted to a small portion of the structure known as the active site. The reactant called substrate is attached to the active site consisting of only a few amino acids. While rest of the bulk of the amino acids maintains the globular structure of the enzyme.
Some enzymes consist solely of proteins. Others also have a non-protein part known as a co-factor, which is essential for the proper functioning of the enzymes. The cofactor usually acts as “bridge” between the enzyme and its substrate, often it contributes directly to the chemical reactions which bring about catalysis. Sometimes the co-factor provides a source of chemical energy, helping to drive reactions which would otherwise be difficult of impossible. Some enzymes use metal ions as co-factors like etc. the detachable co-factor is known as an activator if it is an inorganic ion (fig. 3.1).
Fig. 3.1: Substrate molecules will not fit correctly at the active centre and there will be no catalytic action unless the cofactor molecule is also present.
If the non-protein part is covalently bonded, it is known as a prosthetic group. If it is loosely attached to the protein part it is known as coenzyme. It is closely related to vitamins, which represent the essential raw materials from which coenzymes are made. Only small quantities of vitamins are needed because, like enzymes, co-enzyme can be used again and again. An enzyme with its coenzyme, or prosthetic group, removed is designated as apoenzyme. Adding the correct concentrated coenzyme to the apoenzyme will restore enzyme activity. An activated enzyme consisting of polypeptide chain and a cofactor is known as holoenzyme.
Many enzymes are simply dissolved in the cytoplasm. Other enzymes are tightly bound to certain subcellular organells. They are produced by living cells for use in or near the site of their production. The enzymes important in photosynthesis are found in the chloroplasts and enzymes involved in cellular respiration are found in the mitochondria. Some of the enzymes involved in the synthesis of proteins are integral part of ribosomes.