Enzymes Activities, Properties, Regulation and Physiology

The primary and overriding interest of enzymes, however, is their connection with life. Of all the multitudinous chemical processes in the living cell on which its life depends, there is scarcely one which is not due to enzyme catalysis there can be no life without enzymes. 

Some RNA molecules are now known to possess catalytic activity and have been termed ribozymes. 

The thermodynamic principles described in Chapter 2 of this volume can be used to indicate whether or not a reaction can take place spontaneously. They do not, however, provide information about the rate at which a reaction will proceed. Most biochemical reactions proceed so slowly at physiological temperatures that catalysis is essential for the reactions to proceed at a satisfactory rate in the cell. 

The catalysts are enzymes, most of which are proteins. Not only are they catalysts, bnt their catalytic powers are enormous they can increase the rate of a reaction by several orders of magnitnde. Indeed, in the absence of enzymes, life as we know it wonld not be possible. One remarkable example of the nse of the catalytic power of two enzymes in biology is the Bombardier beetle it uses the enormous catalytic power of the enzymes catalase and a peroxidase to deter predators (Box 3.1). 

Enzymology is therefore central to biochemistry and many books, chapters in books and review articles have been written on this topic. The aim of this chapter is to provide the reader with sufficient information to understand the key aspects of enzymology, how enzymes maintain the life of a cell and help it to perform its physiological fnnctions that contribnte to the life of a human and, finally, to show how individnal enzymes can provide quantitative information on certain aspects of biochemistry and physi- 

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CH 3 ENZYMES ACTIVITIES, PROPERTIES, REGULATION AND PHYSIOLOGY Tangent at zero time... 

Covalent modHication of enzymes, enzyme modulation, enzyme interconversion Oligomeric (i.e. multichain) enzymes may exist in two or more forms, which are interconvertible by enzyme-catalysed covalent modifications, and which differ in their catalytic properties, e.g. activity, substrate affinity and dependence on effectors. Usually the difference in activity is such that one form is active and the other inactive. The activities of the conversion enzymes are in turn regulated by other enzymes, metabolites and/ or effectors. Covalent modifications are therefore important in physiological regulation, in addition to Allostery (see). Whereas allostery provides fine adjustment of metablic rates, C provides an on/off switching of cellular functions, which is very sensitive to environmental influences. 

As has already been mentioned above, the extraction and purification of enzymes separate them from their physiological allosteric effectors. Thus it is not surprising that in vitro studies on dilute purified enzymes did not show these properties. It is only since enzymologists have become interested in the mechanisms of regulation of metabolic activities that they have progressively reconstituted the normal environment of enzymes and thus discovered fundamental phenomena which, up till then, had escaped their attention. 

Thrombin (factor Ha) is the last enzyme protease involved in the coagulation cascade, and it converts fibrinogen to insoluble fibrin that forms the fibrin gel either in physiological conditions or in a pathological thrombus (28). Thrombin has also hormonelike properties, and it is involved in thrombosis and platelet activation. Therefore, thrombin plays a central role in a number of cardiovascular diseases (29), and it is thought to regulate many processes in inflammation and tissue repair at the vessel wall. 

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