1 Enzymology INTRODUCTION 2006/09/18 Downloaded from

2 Aims of this book To give a broad account of enzymology and put current knowledge into perspective. Follow a progression from the properties of isolated enzymes to the behaviour of enzymes in increasingly complex systems, leading to the cells. Downloaded from

3 Historical aspects Enzymes are catalysts which speed up the rates of reactions without themselves undergoing any permanent change. In a cell, there are a large number of enzymes to catalyze all kinds of metabolic reactions. How many enzymes in a cells? It depends on the cells, e.g. about 1700 enzymes are present in E. coli. Enzyme is derived from the Greek meaning ‘in yeast’ and was first used by Kühne in In 1897, Bücher demonstrated that filtrates of yeast of enzymes could catalyze fermentation. Downloaded from

4 Historical aspects In 1894 onward, Fischer proposed ‘lock and key’ hypothesis to explain enzyme specificity. Downloaded from

5 Historical aspects The first enzyme to be crystallized was urease by Sumner in 1926, an enzyme catalyzes the hydrolysis of urea to yield carbon dioxide and ammonia. The development of ultracentrifuge by Svedberg in 1920’s provided very high centrifugal fields for sedimentation of macromolecules. In 1960 the amino-acid sequence of ribonuclease was deduced by Hirs. In 1965 the 3-dimensional structure of lysozyme was deduced by the technique of X-ray crystallography. Downloaded from

6

7 Historical aspects In 1958 Koshland proposed the ‘induced fit’ theory to account for the catalytic power and specificity shown by enzymes. In 1963, Monod and his colleagues postulated ‘allosteric model’ of enzyme control mechanism. In last ten years, the application of recombinant DNA techniques for the study of enzymes has produced some remarkable new insights. In 1986 Cech discovered that RNA can also act as a catalyst for reactions involving hydrolysis of RNA and so called ribozymes. Downloaded from

8

9 Remarkable properties of enzymes as catalysts Catalytic power Specificity Regulation Downloaded from

10 Catalytic power Downloaded from

11 Specificity The range of specificity varies between enzymes. Some enzymes have low specificities, e.g. certain peptidases, phosphatases and esterases, degradation enzymes. Many enzymes show absolute or near-absolute specificity in which they will only catalyze with a single substrate, biosynthetic enzymes. Another distinct feature of many enzyme- catalyzed reactions is their stereospecificity. Downloaded from

12 Downloaded from

13 Downloaded from

14 Downloaded from

15 Regulation Enzymes may be regulated by small ions or other molecules, or by small changes in their covalent structure. Another common phenomenon in many biosynthetic pathways is the feedback inhibition. Downloaded from

16 Cofactors Cofactors are the non-protein components which are required for enzyme activities. One group of cofactors is the metal ions. The second major class of cofactors is the organic cofactors. Tightly bound cofactors are called prosthetic group: –Holoenzyme, enzyme containing a cofactor or a prosthetic group together. –Apoenzyme, the cofactor is removed from the enzyme protein. Downloaded from

17 Downloaded from

18 Downloaded from

19 Downloaded from

20 Nomenclature and classification of enzymes General classification Isoenzymes Multienzyme systems Downloaded from

21 General classification The trivial names are given to many enzymes which consist of the suffix’-ase’ added to the substrate acted on, e.g lactate dehydrogenase, protein kinase. Some trivial names are not helpful, e.g. catalase, papain, trypsin, rhodanese, etc. The present-day accepted nomenclature of enzymes is that recommended by the Enzyme Commission which was set up in 1955 by the International Union of Biochemistry (now known as the International Union of Pure and Applied Chemistry). Enzymes are named according to certain well-defined rules. Downloaded from

22 General classification The six major types of enzyme-catalyzed reactions are: 1.Oxidation-reduction reactions, catalyzed by oxidoreductases. 2.Group transfer reactions, catalyzed by transferases. 3.Hydrolytic reactions, catalyzed by hydrolases. 4.Elimination reactions in which a double is formed, catalyzed by lyases. 5.Isomerization reactions, catalyzed by isomerases. 6.Reactions in which two molecules are joined at the expense of an energy source (usually ATP), catalyzed by ligases. Downloaded from

23 General classification Enzymes are further classified by being assigned an Enzyme Commission (EC) number consisting four parts (a, b, c, d). The first number (a) indicates the type of reaction catalyzed and the numbers are taken from 1 to 6. The second number (b) indicates the subclass, which usually specifies the type of substrate or the bond cleaved more precisely. The third number (c) indicates the sub-subclass, allowing an even more precise definition of the reaction catalyzed. The fourth number (d) indicates the serial number of the enzyme in its sub-subclass. Downloaded from

24 General classification Examples of EC numbers of enzymes: – alcohol dehydrogenase – lactate dehydrogenase – hexokinase – adenosinetriphosphatase – fructose-bisphophate aldolase – triosephosphate isomerase – isoleucine-tRNA ligase See Appendix, pp450 Downloaded from

25 Isoenzymes Enzymes existing several forms of enzyme catalyzing the same reaction, and the differences are in term of their amino-acid sequences within a single species. One of the example is lactate dehydrogenases in heart and muscle. Downloaded from

26 Multienzyme systems Multienzyme systems are proteins that exhibit more than one catalytic activity. Downloaded from

27 The contents of this book Chapter 1, the introduction. Chapter 2 to 6, discussing the behaviour of isolated enzymes (2), structural characterization (3), kinetics (4), catalytic action (5) and control of activity (6). Chapter 7 and 8 include the multienzyme systems and enzymes in the cells. Chapter 9, on enzyme turnover. Chapter 10 and 11, applications of enzymes. Downloaded from