Introduction of Enzyme , enzyme as catalysts, Functional enzymes and non-functional Enzyme

INTRODUCTION OF ENZYME

Enzymes are synthesized by the cells of all living organisms. They act like catalysts and accelerate the multitude of metabolic reactions upon which life depends. Without exception, all enzymes are proteins and their catalytic activity depends on the presence of a precise conformational structure in the folded polypeptide chains. Even minor alterations in this structure may result in the loss of activity. Enzymes act on specific substrates. Substrate is a substance on which an enzyme acts and a specific end product/s is/are produced. For example, when amylase (enzyme) acts on starch (substrate) the end products produced are maltose and dextrins. Without enzymes life would not be possible. As the biocatalysts, enzymes occupy central role in health and disease. In health, all physiologic processes occur in an ordered, regulated manner mainly due to the catalytic functions of enzymes, which coordinate numerous metabolic reactions inside the cells. Enzymes play very important role as catalysts to carry out processes such as digestion, absorption, acid base balance, respiration, synthesis of various proteins, lipoproteins and hormones and several other anabolic reactions, energy providing catabolic reactions, neurotransmission, coagulation and defense mechanisms, reactions involving transportation of various substances and muscular activities. Homeostasis can be profoundly disturbed in pathological states which can impair the ability of cells to form the enzymes, which catalyze important metabolic processes. For example, in liver cirrhosis, since cells are unable to synthesize key enzymes responsible to convert ammonia into urea, ammonia intoxication occurs. Several genetic diseases are linked with the absence of certain important enzymes, which participate in the synthesis of metabolites.

Following severe tissue injury (severe liver disease, cardiac or lung infarct) or uncontrolled cell growth (e.g. prostatic carcinoma), specific tissue enzymes are released in to blood. Measurement of these intracellular enzymes in serum provides physicians with valuable diagnostic and prognostic information.

In diagnostic technology, it was possible to introduce mono-step methods which involve use of enzymes in the reagents. These enzyme based reagents are nontoxic, noncorrosive and very safe to handle. Both manual and automated methods can be performed by using reagents which contain enzymes. For example, diagnostic kits, such as, glucose-oxidase, urease, uricase etc. are used for the determination of serum (plasma) glucose, urea and uric acid respectively. The ability of enzymes to conjugate with other proteins without significant loss of catalytic activity, has been used in various types of ELISA methods. The enzyme labels are used instead of radioactive isotopes in ELISA  echniques for the determination of various hormones, antibodies and drugs in a clinical chemistry laboratory. (for ELISA methods Refer Chapters 4 & 36).

In living organisms, enzymes are rapidly degraded and their supply is replenished by new synthesis. Over 700 enzymes have been isolated in a pure or crystalline form. Some enzymes are relatively small molecules with molecular weights in the order of 10,000, whereas other are very large molecules with molecular weights ranging from 150000 to over a million.

ENZYMES AS CATALYSTS

A chemical catalyst may be defined as a substance which speeds up the rate of a particular chemical reaction without Itself being consumed or permanently altered in the catalytic process. Enzymes are special chemical catalysts of biological origin. Only a small quantity of catalyst is needed to exercise its effect on a reaction and this small quantity of catalyst can carry on its catalytic role over and over again. Thus, the mass of reactants consumed and products formed is many times greater than the mass of the catalyst present. The enzymes differ with the inorganic catalysts such as manganese dioxide, (used in the preparation of oxygen) and platinum (used in the preparation of sulfuric acid) in the following ways:

Functional Enzymes

 Plasma active enzymes: These enzymes are present at all times in the circulation of normal individuals. Their substrates are also present in the circulation, either continuously or intermittently. The 'functional' enzymes, which perform a physiologic function in blood, include the proenzymes of blood coagulation (thrombokinase) and of blood clot dissolution, lipoprotein lipase and pseudocholinesterase etc. They are generally synthesized in the liver, but are present in equivalent or higher concentrations than in tissues.

Nonfunctional Enzymes

These enzymes perform no known physiologic function in blood. Their substrates frequently are absent from plasma. The enzymes are present in the blood of normal individuals at levels upto a million fold lower than in tissues. These enzymes are (a) The enzymes of exocrine secretions and (b) Intracellular enzymes. The nonfunctional enzymes enter the plasma in small amounts due to the continuous ageing of cells or due to the diffusion through damaged cell membrane. The low levels of nonfunctional enzymes found ordinarily in plasma arise apparently from the routine, normal destruction of erythrocytes, leukocytes and other cells. Vigorous exercise also results in the release of small quantities of muscle enzymes.