Enzymes-1 PDF

ENZYMES 1 Sahar Abdelmoneim  Learning objectives: By the end of this lecture all students should be able to:  Define enzymes.  Classify enzymes and understand the enzyme nomenclature.  Know the role of coenzymes.  Discuss enzymes activity and specificity.  Explain the enzymes-substr...

ENZYMES 1 Sahar Abdelmoneim  Learning objectives: By the end of this lecture all students should be able to:  Define enzymes.  Classify enzymes and understand the enzyme nomenclature.  Know the role of coenzymes.  Discuss enzymes activity and specificity.  Explain the enzymes-substrate interaction.  Enzymes are biological catalysts Almost every significant life process is dependent on enzyme activity.. Almost all enzymes are protein, except for a class of RNA modifying catalysts known as ribozymes. 3  In enzyme-catalyzed reactions, substrate concentrations are usually greater than the enzyme concentration. Enzyme catalysis involves the binding of the substrate to a specific site on the enzyme, (active or catalytic site). 4  During enzyme catalysis, (ES) complex is first formed then; proceeds to a transition state (ES*); before it forms enzyme product complex (EP) which dissociate to product and free enzyme. The series of events can be shown thus: E + S ES ES* EP E + P 5  Traditionally, enzymes were simply assigned names by the investigator who discovered the enzyme. As knowledge expanded, systems of enzyme classification became complex. 6  Currently enzymes are grouped into six functional classes by the International Union of Biochemists (I.U.B.). I.U.B. gave each enzyme a unique number. And specifies a textual name for each enzyme 7 No. Class Biochemical Properties 1 Oxidoreductases Add or remove hydrogen atoms. 2 Transferases Transfer functional groups. 3 Hydrolases Add water across a bond, hydrolyzing it. 4 Lyases Add or remove water, ammonia or carbon dioxide across double bonds 5 Isomerases Catalyze isomerizations, 6 Ligases Join two chemical groups with the use of energy from ATP. 8  Each enzyme is given a four digit number specifying its class, subclass, substrate and the type of the reaction The enzyme's name is comprised of the names of the substrate (S), the product (P) and the enzyme's functional class. In everyday usage, most enzymes are still called by their common (trivial) name. 9  Enzymes are also classified on the basis of their composition. Enzymes composed wholly of protein are known as simple enzymes in contrast to complex enzymes, which are composed of protein plus a relatively small organic molecule. Complex enzymes are also known as holoenzymes. 10  The protein component in the holoenzyme is called apoenzyme, while the non-protein part is the coenzyme or prosthetic group. The prosthetic group is the small organic molecule bound to the apoenzyme by covalent bonds Coenzyme is the small organic molecule non- covalently bound to the apoenzyme 11  The non-protein component of an enzyme may be as simple as a metal ion or as complex as a small non- protein organic molecule Many prosthetic groups and coenzymes are water- soluble derivatives of vitamins. Enzymes that require a metal in their composition are known as metalloenzymes 12  Coenzymes act as transporters of chemical groups from one reactant to another. The chemical groups carried can be as simple as the hydride ion (H+ + 2e-) carried by NAD or the mole of hydrogen carried by FAD; or amine (-NH2) carried by pyridoxal phosphate. 13  Coenzymes are chemically changed as a consequence of enzyme action, thus can be considered as second substrates, Unlike usual substrates the coenzymes are recycled to their original form when donate the carried chemical grouping to an acceptor molecule 14  Enzymes are highly specific for the kind of reaction they catalyze, Enzymes are also specific towards the substrate, Some enzymes have broad substrate specificity, eg. Alcohol dehydrogenase and hexokinase 15  Enzymes also are generally specific for a particular steric configuration (D and L optical isomer) of a substrate. The racemases are striking exception to these generalities; they convert D isomers to L isomers and vice versa 16  Are multiple forms of an enzyme acting on the same substrate and produce the same product. These are the products of genes that vary only slightly. Various isozymes are expressed in different tissues of the body. 17  The best studied set of isozymes is the lactate dehydrogenase (LDH). LDH is a tetrameric enzyme composed of all possible arrangements of two different protein subunits (H) for heart and (M) for skeletal muscle. 18  HHHH Isoenzyme-1 Heart HHHM Isoenzyme-2 HHMM Isoenzyme-3 HMMM Isoenzyme-4 MMMM Isoenzyme-5 Muscle 19  Two models were proposed to explain the nature of enzyme substrate binding. The Key and lock model The induced fit model: 20 The induced fit model proposes that the initial interaction between enzyme and substrate is relatively weak, and not necessarily complementary but it induces conformational changes in the enzyme 21 THANKS