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.