Enzymes:
Enzymes are 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 them the reaction would proceed at very slow speed making life impossible. They 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 structure known as active site. The reactant called substrate is attached to active site consisting of only few amino acids while rest of bulk of amino acids maintains the globular structure of enzyme. Some of them consist solely of proteins. Others also have a non protein part known as co-factor which is essential for proper functioning of enzymes. Cofactor usually acts as bridge between enzyme and its substrate often it contributes directly to chemical reactions which bring about catalysis. Sometimes cofactor provides source of chemical energy helping to derive reactions which would otherwise be difficult or impossible. Some enzymes use metal ions as cofactor. The detachable cofactor is known as an activator if it is an inorganic ion.
If non-protein part is covalently bonded it is known as prosthetic group. If it is loosely attached to protein part it is known as coenzyme. It is closely related to vitamins which represent essential raw materials from which coenzymes are made. Only small quantities of vitamins are needed because like enzymes coenzyme 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 apoenzyme will restore enzyme activity. An activated enzyme consisting of polypeptide chain and cofactor is known as holoenzyme. Many enzymes are simply dissolved in cytoplasm and others are tightly bound to certain subcellular organelles. They are produced by living cells for use in or near the site of their production.
Characteristics of enzymes:
All enzymes are globular proteins. They increase the rate of reaction without themselves being used up. Their presence does not effect nature or properties of end product. Small amount of an enzyme can accelerate chemical reactions. They are very specific in their action a single enzyme catalyzes only single chemical reaction or group of related reactions. They are sensitive to even a minor change in PH, temperature and substrate concentration. Some of them require a cofactor for their proper functioning. They lower the activation energy of reactions.
Mechanism of enzyme action: An enzyme is a three dimensional globular protein that has specific chemical composition due to its component amino acids and a specific shape. Every enzyme by virtue of its specificity recognizes and react with a special chemical substances called substrate. They therefore reacts only with its specific substrate and transforms it into product. It is then released unaltered and thus can be used again and again. In certain cases they act in a series of chemical reactions in a particular order to complete a metabolic pathway such as respiration and photosynthesis. The successive enzymes containing these reactions are normally present together in a precise order of reaction such that substrate molecules can be literally handed on from one enzyme to another forming a enzyme to enzyme chain. In this way the products from one step in pathway are transferred to the enzyme catalyzing the next step. An enzyme and its substrate react with each other through a definite charge bearing site called active site. The charge and shape of active site is formed by some amino acids present in polypeptide chain of active site. These amino acids are brought closer and are arranged in specific way by coiling and folding of polypeptide chain within globular symmetry of enzyme. Active site is made up of two definite regions the binding site and catalytic site.
Binding site helps enzyme in recognition and binding of a proper substrate to produce ES complex. This reaction activates catalytic site. Activated catalytic site catalyzes the transformation of substrate into product. Thus the enzyme after catalysis detaches itself from the products unchanged. They require aqueous media for its activity.
Emil Fischer(1890) proposed a Lock and Key model to visualize substrate and enzyme interaction. According to this model as one specific key can open only a specific lock in the same manner a specific enzyme can transform only one substrate into product. According to Lock and Key model active site is a rigid structure. There is no modification or flexibility in active site before during or after the enzyme action and it is used only as template. Later studies did not support this model in all reactions.
On the basis of new evidences Koshland (1959) proposed Induce Fit model. He argued that when a substrate combines with an enzyme it induces changes in enzyme structure. The change in structure enables the enzyme to perform its catalytic activity more effectively.
0 Comments