ENZYMES. Definition These are organic substance that accelerates the rate of chemical reactions. Organic catalyst are enzyme: 1.Highly specific: catalyze.

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 10 17 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.Oxidoreductases2.Transferases3.Hydrolases4.Lyases5.Isomerases6.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

IsomerasesTransfer of groups within molecules to yield

isomeric forms

one substrate and one product are involved,

LigasesFormation 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,enzymeconcentration.

•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 –k1ES

• 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(km) .

• 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

MethotrexateA 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 bind @ 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.