Enzymes basics

Enzymes are biological catalysts—i. e substances of biological origin that accelerate chemical reactions (see p. 24). The orderly course of metabolic processes is only possible because each cell is equipped with its own genetically determined set of enzymes. It is only this that allows coordinated sequences of reactions (metabolic pathways see p. 112). Enzymes are also involved in many regulatory mechanisms that allow the metabolism to adapt to changing conditions (see p.ll4). Almost all enzymes are proteins. However, there are also catalytically active ribonucleic acids, the ribozymes (see pp. 246, 252). 

The catalytic action of an enzyme, its activity, is measured by determining the increase in the reaction rate under precisely defined conditions—i.e., the difference between the turnover (violet) of the catalyzed reaction (orange) and uncatalyzed reaction (yellow) in a specific time interval. Normally, reaction rates are expressed as the change in concentration per unit of time (mol 1 s see p. 22). Since the catalytic activity of an enzyme is independent of the volume, the unit used for enzymes is usually turnover per unit time, expressed in katal (kat, mol s ). However, the international unit U is still more commonly used (pmol turnover min 1 U = 16.7 nkat). 

The action of enzymes is usually very specific. This applies not only to the type of reaction being catalyzed (reaction specificity), but also to the nature of the reactants ( substrates ) that are involved (substrate specificity see p.94). In Fig. B, this is illustrated schematically using a bond-breaking enzyme as an example. Highly specific enzymes (type A, top) catalyze the cleavage of only one type of bond, and only when the structure of the substrate is the correct one. Other enzymes (type B, middle) have narrow reaction specificity, but broad substrate specificity. Type C enzymes (with low reaction specificity and low substrate specificity, bottom) are very rare. 

More than 2000 different enzymes are currently known. A system of classification has been developed that takes into account both their reaction specificity and their substrate specificity. Each enzyme is entered in the Enzyme Catalogue with a four-digit Enzyme Commission number (EC number). The first digit indicates membership of one of the six major classes. The next two indicate subclasses and subsubclasses. The last digit indicates where the enzyme belongs in the subsubclass. For example, lactate dehydrogenase (see pp. 98-101) has the EC number 1.1.1.27 (class 1, oxidoreductases subclass 1.1, CH-OH group as electron donor sub-subclass 1.1.1, NAD(P) as electron acceptor). 

Enzymes with similar reaction specificities are grouped into each of the six major classes  

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The biochemical network is built of a number of processing elements (i.e., the biochemical neurons). These are the enzymic basic systems. The term elementary is not an absolute one. However, the processing based on a few enzymic reactions is less complex than the processing of electrical signals as achieved by natural nerve cells. 

The enzymic basic systems can be highly interconnected, due to chemical components that participate in processes that take place in more than one biochemical neuron. 

Fluorescence-based assays either in the measurement of enzyme activity or in the quantification of enantioselectivity all have a high degree of sensitivity, which allows the use of very dilute substrate concentrations and extremely small amounts of enzymes. Basically, there are two different approaches. One involves the use of a substrate of interest to which a fluorescent-active (or potentially active) moiety is covalently attached. The second approach makes use of a fluorescence-based sensor, which gives rise to a signal as a consequence of the enzyme-catalyzed reaction of a substrate of interest. 

Enzyme kinetics is studied for two reasons (1) it is a practical concern to determine the activity of the enzyme under different conditions (2) frequently the analysis of enzyme kinetics gives information about the mechanism of enzyme action. Chapter 7, Enzyme Kinetics, begins with an introductory section on the discovery of enzymes, basic enzyme terminology and a description of the six main classes of enzymes and the reactions they catalyze. The remainder of the chapter deals with basic aspects of chemical kinetics, enzyme-catalyzed reactions and various factors that affect the kinetics. 

Stryer, L. (1995). Enzymes Basic Concepts and Kinetics. In Biochemistry, 4th ed. New York Freeman. 

This book provides an introduction to general enzymology for the industrial enzymologist and then deals, chapter by chapter, with enzymes or application areas specific to food technology. For each enzyme, basic data are provided on chemistry, kinetics, sources, pH optima, and the like. In addition, specific food application data are given. Production and legal aspects are also mentioned. This is a How to Use Enzymes in Foods book. 

The adsorption of an enzyme onto a support or film material is the simplest method of obtaining an immobilized enzyme. Basically, the enzyme is attached to the support material by noncovalent linkages and does not require any preactivation step of the support. The interactions formed between the enzyme and the support material will be dependent on the existing surface chemistry of the support and on the type of amino acids exposed at the surface of the enzyme molecule. Enzyme immobilization by adsorption involves, normally, weak interactions between the support and the enzyme such as ionic or hydrophobic interactions, hydrogen bonding, and van der Waals forces (see Figure 44.1). ... 

Knott, P. J., Hutson, P. H., Scraggs, R. P., and Curzon, G., 1981, Electrochemical recording of brain catecholamine and serotonin release during behavioral changes, in Function and Regulation of Monoamine Enzymes Basic and Clinical Aspects (E. Usdin, N. Weiner and M. B. H. Youdin, eds.), pp. 771-780, Macmillan, New York. 

Cytochrome P450 enzymes are the most widespread, active, and most versatile in their xenobiotic Phase I transformation activity. These enzymes are composed of heme-containing enzymes in the ferric ion state. In transformations the ferric ion is reduced to the ferrous ion that can bind Oj and CO. These enzymes basically add oxygen or remove hydrogen in a step-wise process to generate Phase I biotransformation products. Most cytochrome P450 transformations require an additional enzyme (co-enzyme) to assist in the transfer of electrons. Cytochrome P-450 enzymes carry out many kinds of oxidations - hydroxylations, epoxidations, heteroatom oxidations, N-hydroxylations, dealkylations, ester hydrolysis, and dehydrogenation. 

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