ENZYMES

Definition These are organic substance that accelerates the rate of chemical reactions. Organic catalyst are enzyme: 1.Highly specific: catalyze one or two reactions only. 2.Protein in nature: denatured by heat. Inorganic catalyst are metals: 1.Non-specifin:catalize many reactions 2. Not affected by heat.

the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, the products.

Enzymes Have a recommended name Suffix –”ase” attached to the substrate of the reaction e.g. glucosidase, ”ambreenase”, “vishaalase” OR to describe the action performed. e.g. lactate dehydrogenase

4-types of specificity Optical specificity: acts on one of 2 isomers. e.g maltase acts on α- glycosidase and not β-glycosidase. Group specificity: the presence of certain group.e.g pepsin acts on peptide bonds. Absolute specificity: only on one substrate e.g urease acts only on urea. Relative specificity: acts on group of compound having same type of bonds.lipase acts on different triglycerides.

Enzyme activity Enzymes : simple or conjugated. Simple: native conformation of protein. Conjugated protein: holoenzyme. Apoenzyme + cofactor-> holoenzyme.

..truly amazing substances Active sites: special pocket where substrate binds (reaction occurs) Catalyze reaction 10 3 TO TIMES FASTER than uncatalyzed reactions Are highly specific i.e. can catalyze only one type of reaction Some enzymes require an additional chemical component-COFACTOR e.g. metal ions such as Zn 2+, Fe 2+, Mg 2+

..still on properties or……….an organic molecule called a coenzyme e.g. NAD+ contains Niacin, FAD contains riboflavin Holoenzyme - enzyme with its cofactor Apoenzyme -protein portion of the holoenzyme. Apoenzyme shows no biologic activity without appropriate cofactor

Zymogens Some enzyme are synthesized as inactive forms called zymogen or proenzyme e.g trypsinogen and pepsinogen. 1. Zymogen are inactive because their catalytic sites are masked by a polypetide chain. 2. To activate, cleavage of polypeptide chain.

IUBMB Systematic name Enzymes are divided into 6 major classes Suffix “–ase” is attached to describe the chemical reaction catalyzed

1.Oxidoreductases 2.Transferases 3.Hydrolases 4.Lyases 5.Isomerases 6.Ligases

Oxidoreductases Enzymes catalyzes an oxidation-reduction reaction between two substrate. S(oxidised)+Y(reduced)  s(reduced) + Y (oxidised).

Transferases Enzyme catalyzes the transfer of a group other than hydrogen from one substrate to another. SX + Y  S + YX Glucose +ATP------glucokinase  gucose-6-phosphate +ADP.

Hydrolases Catalyzes hydrolysis. A –B --HOH----  AH +BOH peptidase

Lysases Catalyzes removal of group from substrate by mechanism other than hydrolysis. Decarboxylase. Pyruvate  acetladehyde + CO2

Isomerases Transfer of groups within molecules to yield isomeric forms one substrate and one product are involved,

Ligases Formation of C-C, C-S,C-O and C-N bonds coupled to hydrolysis of high energy phosphates e.g. ATP Often referred to as SYNTHETASE

LOCK AND KEY HYPOTHESIS

Induced fit theory

Mechanism of enzyme action Free energy of the reaction: initial state to final state, consume energy. Activation energy: absorb energy (activated state or transition state). The effect of enzyme is to decrease the energy of activation.

More properties Enzyme activity can be regulated i.e. activated or inhibited so rate of product formation suits demands of the cell Are reusable because they aren't altered in a reaction

Factors affecting reaction velocity Substrate concentration,enzyme concentration. Temperature pH

Concentration of enzyme The amount of enzyme is in a reaction is doubled,the amount of substrate is converted to product is doubled.

Concentration of substrate At low substrate concentration,not all enzyme are saturated.So the rate of reaction will increase.(V max) At higher substrate concentration,all enzyme molecules get saturated with substrate and any more increase of substrate concentration will result in no increase.

Michaelis –menten equation It describes the dependence of reaction velocity on substrate concentration. E + S –k1  ES ES breaks down to give enzyme and product. E + S K-1 E+ S K2 E + P So, K1, K-1,and k2(rate constants)

Cont. Vi=Initial velocity Vmax =all enzyme are involved in an ES complex. [s]= increased Substrate Vi=Vmax [s] [s] + (k1+k2) K-1

Cont. The ratio constants k1 + k2/k-1 as michaelis constant(k m ). So km=k1+ k2 k-1 then vi = Vmax [s] [s] + km Michael equation=km When substrate conc.[s] is equal to km Vi =vmax[s] = vmax [s] =Vmax [s] + km 2S 2

Cont. Thus km can be defined as substrate that produce half maximum velocity.

Effect of temperature The optimal temp. for enzyme activity in human body is that temp similar of that the cells(37˚) At zero,enzyme inactive The velocity is almost doubled every 10˚C At 55˚-60˚C,most enzyme are denatured and become permanenlty inactive.

Effect of pH The optimal pH activity is that pH at which the enzyme acts maximaly. Above and below, the reaction decline. Each enzyme has has its own optimal pH.e.g pancreatic lipase 7.5 Extream of pH leads to denaturation.

Enzyme activators To activate the enzyme Metal ions; Chloride ions which activates salivary amylase and calcium ions which activate blood clotting enzymes. Zymogen needs other enzyme for activation

Inhibition of enzyme activity Any substance that can diminish the velocity of an enzyme-catalyzed reaction-INHIBITOR Is Reversible (through non-covalent bonds) or Non-reversible (through covalent bonds) TYPES Competitive: reversible binding to the same site for the substrate and competes actively for the site.

40 Methotrexate A competitive inhibitor of dihydrofolate reductase - role in purine & pyrimidine biosynthesis Used to treat cancer

statin drugs competitively inhibit 1st committed step in cholesterol synthesis catalyzed by HMG CoA Reductase e.g. latorvastatin (Lipitor)-↓ plasma cholesterol levels. Non-competitive: inhibitor and substrate diff sites. either free enzyme or ES complex and prevents reaction from running e.g. lead inhibiting ferrochelatase Enzyme inhibitors can be used as drugs e.g. Penicillin, amoxicillin, ACE inhibitors

Allosteric regulation of enzyme activity allosteric enzyme generally catalyze the irreversible steps in metabolic pathways. Allosteric mean “other site”,they bind non- covalently at a sit other than active site.