Definition of Enzymes:
Enzymes are biocatalysts. They speed up the rate of metabolic reactions occurring in living cells. The substance upon which an enzyme acts is called substrate and this substrate is usually converted to product at the end of the enzymecatalysed reaction as represented by the scheme below:
S(Substrate) → (Enzyme) P(Product)
Enzyme Nomenclature and Classification:
Nomenclature of Enzymes:
At times, enzymes are just named by adding suffix '-ase' to substrate. Therefore, lactase acts on lactose, amylase on amylose etc. Usually speaking, there are two kinds of enzyme nomenclatures in common use: trivial or working names and systematic names. Trivial names are usually and adequately shorter for general use, although not essentially very systematic. They are generally allotted arbitrarily. Amylase, trypsin, and lactase are examples of trivial names. You are probable to meet more of these trivial names than systematic names in further reading. Systematic name of the enzyme, in contrast, shows action of enzyme as exactly as possible, therefore recognizing enzyme precisely. Systematic names are allotted in agreement with specific rules laid down by Enzyme Commission. Systematic names are the ones utilized in enzyme classification. Lactate: NAD+ Oxidoreductase and Xylitol: NAD+ 2-Oxidoreductase is systematic names of Lactate dehydrogenase and D-Xylulose reductase respectively.
Categorization of Enzymes:
Enzymes on basis of reactions they catalyse. There are 6 main reaction types/classes to which 6 main classes of enzymes have been recognized. These comprise;
i) Oxidoreductases - catalyze transfer of hydrogen.
ii) Transferases - catalyze transfer of groups other than hydrogen.
iii) Hydrolases - these cleave bonds and add water.
iv) Lyases - cleave without adding water.
v) Isomerases - intramolecular transfers.
vi) Ligases - catalyse ATP-dependent condensation of two molecules.
Enzymes are allotted code numbers (also known as enzyme commission numbers). These code numbers, prefixed by EC, have four elements separated by points, with the given meaning:
i) First number illustrates to which of the 6 main divisions enzyme belongs;
ii) Second figure indicates subclass;
iii) Third figure provides sub-subclass; and
iv) Fourth figure is serial number of enzyme in its subsubclass. For instance code [EC 220.127.116.11] has been allotted to enzyme sialidase, stating that it is hydrolase.
General Properties of Enzymes:
The given properties are feature of enzymes:
E+S ↔ ES →E+P
The main features of active site comprise:
i) The active site is small portion of total volume of enzyme molecule.
ii) It is three-dimensional.
iii) It binds to substrates using comparatively weak forces.
iv) Active sites are clefts or depression where water is excluded. Such clefts have polar residues which are necessary for enzyme activity.
v) Enzymes are definite in kind of reactions they catalyze. For instance, amylase will act on amylose and not glucose. At times, such specificities are group specificities and in some instances stereospecificities.
vi) Enzymes are extremely regulated through balance between their synthesis and degradation. For example, at gene level their production can be handled by induction and repression mechanisms of coding gene.
vii) Enzymes are sensitive to temperature and are denatured at high temperature.
viii) Enzymes are also sensitive to variation in pH (acidity or alkalinity of environment).
ix) Enzymes can be inactivated by inhibitors.
How Enzymes Work:
Enzyme catalysis occurs within the area in enzyme called as active site. Generally catalysis start with substrate (molecule to be acted upon) getting bound by enzyme at active site. This is followed by lowering activation energy so that the reaction will proceed at a faster rate than it would in the absence of the enzyme.
Binding of substrate to enzyme could be in form of Lock -and - key (where active site assumed the rigid shape that accommodates only substrate with identical shape fits in) or through Induced fit approach in which enzyme suffers a change in conformation when substrate binds. Induced fit theory of substrate binding at active site is most acknowledged theory. Upon binding, substrate gets sequestered entirely from solution forming the enzyme -substrate complex. This complex is extremely central to enzyme action.
Co-enzymes and Vitamins:
Co-enzymes are low molecular weight organic substances that enzymes could need for their catalytic activities. Frequently, co-enzymes are derivatives of vitamins. At times, extra chemical group an enzyme may need is/are one or more inorganic ions. They are therefore known as co-factors. Coenzyme or co-factor which is very tightly (covalently) bound to enzyme protein is a prosthetic group. Whole catalytically active enzyme together with bound co-enzyme and/or metal ion is known as holoenzyme. Protein part of such enzyme is known as apoenzyme or apoprotein.
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