Transcript Enzymes
Enzymes Enzymes • Are biological catalysts • Catalysts are substances that that increase the speed of a chemical reaction by lowering the energy requirement • Although a catalyst influences a chemical reaction, it is not itself permanently changed, nor does it cause the reaction to occur. • A catalyst can increase the rate of a chemical reaction but cannot cause the reaction . Enzymes • Catalysts are not used up so they can be used over and over again. • Enzymes are organic catalysts produced by the body • The reactant in an enzyme catalyzed reaction is called the substrate • The active site of an enzyme is the small portion of the molecule that is responsible for the catalytic action of the enzyme. • Enzyme provide a chemical pathway that has a lower activation energy than that without the enzyme. Enzymes. • Enzymes are highly specific with varying degrees of specificity. • Some enzymes exhibit absolute specificity • That is they act on substrate and only on that substrate. • Some enzymes also exhibit stereo specificity. Such enzymes can detect difference between optical isomers.. mirror images and select only one of such isomers. • Some enzymes are linkage specific i.e they catalyze the reaction that breaks the bond only between specific groups. Enzymes • For example the enzyme thrombin will break the bonds between amino acids arginine and glycine and does not affect bonds between other amino acids. • Other enzymes exhibit reaction specificity i.e. they catalyze certain types of reaction • E.g. Carbohydrases catalyzes the hydrolysis of carbohydrates. • Other enzymes exhibit group specificity e.g. chymotrypsin catalyzes the hydrolysis of only those proteins that contain phenylalanine, tryptophan or tyrosine Regulation • The activities of enzymes is closely regulated . • There are substances in the cells that can increase or decrease the activity of the enzyme and thus control the rate of that particular reaction. • Enzymes are proteins and so undergo the same reactions. • They are coagulated by heat, alcohol, strong acids. Regulation • Temperature • PH • Effects of concentration. OPTIMUM PH Optimum Temperature Activators and inhibitors • Inorganic substances that tend to increase the activity of an enzyme are called activators. • For example, the magnesium ion (Mg2+) is an inorganic activator for the enzyme phosphatase, and zinc ion (Zn2+) is an activator for the enzyme carbonic anhydrase. • An enzyme inhibitor is any substance that will make an enzyme less active or render it inactive. • Enzyme inhibitors that bind reversibly to the active site and so block access by the substrate are called competitive inhibitors. Activators and inhibitors • Their effect can be overcome by increasing the concentration of the substrate. • Other inhibitors that bind to another site on the enzyme to render it less active or inactive are called non competitive inhibitors. • They act by changing the conformation of the enzyme, thereby reducing or stopping its activity. • Irreversible inhibitors form strong covalent bonds with the enzymes rendering it inactive. • This effect cannot be overcome by increasing the concentration of the substate. • Zalcitabin ( DIDEOXYCYTIDINE ) is used in patients with advanced HIV infection. • Inside the cell, Zalcitabin is converted into its active metabolite dideoxycytidine 5’-triphosphate (ddCTP). • This in turn act as a competitive inhibitor of the natural substrate deoxycytidine triphosphate (dCTP) for the active site of the viral reverse transcriptase .. • As a result of this competitive inhibition, the replication of the HIV 1 is inhibited. Activators and inhibitors • Heat changes in PH, heat, strong acids, alcohols can all denature protein. • These are examples of nonspecific inhibitors. • They affect all enzymes in the same manner. • Specific inhibitors affect one single enzyme or group of enzymes • In this category are most poisonous substances, such as cyanide ion (CN) which inhibits the activity of the enzyme cytochrome oxidase. Poisons • Many enzyme inhibitors are poisonous because of their effect on enzyme activity. • Mercury and lead compounds are poisonous because they react with the sulfhydryl group (SH) of an enzyme and so change its conformation. • The subsequent loss of enzyme activity leads to the various symptoms of lead and mercury poisoning such as loss of equilibrium, hearing, sight and touch which are generally irreversible. Poisons • While some enzyme inhibitors are poisonous, others are beneficial to life. • Penicillins act as an enzyme inhibitor for transpeptidase, a substance that bacteria need to build their cell walls. • If the cell wall is lacking, osmotic pressure causes the bacteria cell to burst and die. • Cyclic AMP acts a chemical messanger to regulate enzyme activity within the cells that store carbohydrates and fat. • Without cAMP, the activity of all enzymes working at maximum speed within the cell would soon create chaos. Mode of Activity • Each enzyme contains an active site. • Active site is that section of the molecule at which combination of the substrate takes place. • The active site consists of different parts of the protein chain (the enzyme) • These parts are brought together by the folding and bending of the protein chain (the secondary and tertiary structures) Mode of Activity • So that the active site occupies a relatively small area. • The fact that enzymes can be denatured by heat indicates the importance of structural arrangement. • It is believed that enzyme activity occurs in 2 steps. • First the active site of the enzyme combines with the substrate to form an enzyme – substrate complex Lock and key • The Enzyme – substrate complex then breaks up to form the product and the free enzyme which can react again • This theory is known as the LOCK AND KEY THEORY. • According to this theory, the substrate must ‘’FIT” into the active site of the enzyme. • This accounts for the Specificity of the enzyme. Lock and Key Induced – Fit model • A more recent version of the activity of an enzyme ..the induced fit model suggests that the active site is not rigid , as in the lock and key model but flexible. • That is the site changes in conformation upon binding to a substrate in order to yield an enzyme-substrate fit. Induced Fit model