enzymeslibvolume7.xyz/.../enzymes/enzymespresentation1.pdf · · 2015-01-06enzyme controlled...

ENZYMES

A protein with catalytic properties due to its

power of specific activation

© 2007 Paul Billiet ODWS

Chemical reactions

� Chemical reactions need an initial input of energy =

THE ACTIVATION ENERGY

� During this part of the reaction the molecules are

said to be in a transition state.


Reaction pathway


Making reactions go faster

� Increasing the temperature make molecules move

faster

� Biological systems are very sensitive to temperature

changes.

� Enzymes can increase the rate of reactions without

increasing the temperature.

� They do this by lowering the activation energy.

� They create a new reaction pathway “a short cut”


An enzyme controlled pathway

� Enzyme controlled reactions proceed 108 to 1011 times faster

than corresponding non-enzymic reactions.© 2007 Paul Billiet ODWS

Enzyme structure

� Enzymes are proteins

� They have a globular shape

� A complex 3-Dstructure

Human pancreatic amylase

© Dr. Anjuman Begum


The active site

� One part of an enzyme,

the active site, is

particularly important

� The shape and the

chemical environment

inside the active site

permits a chemical

reaction to proceed

more easily© H.PELLETIER, M.R.SAWAYA ProNuC Database


Cofactors� An additional non-

protein molecule that is needed by some enzymes to help the reaction

� Tightly bound cofactors are called prosthetic groups

� Cofactors that are bound and released easily are called coenzymes

� Many vitamins are coenzymes Nitrogenase enzyme with Fe, Mo and ADP cofactors

Jmol from a RCSB PDB file © 2007 Steve Cook

H.SCHINDELIN, C.KISKER, J.L.SCHLESSMAN, J.B.HOWARD, D.C.REESSTRUCTURE OF ADP X ALF4(-)-STABILIZED NITROGENASE COMPLEX AND ITS

IMPLICATIONS FOR SIGNAL TRANSDUCTION; NATURE 387:370 (1997)


The substrate

� The substrate of an enzyme are the reactants

that are activated by the enzyme

� Enzymes are specific to their substrates

� The specificity is determined by the active

site


The Lock and Key Hypothesis

� Fit between the substrate and the active site of the enzyme is exact

� Like a key fits into a lock very precisely

� The key is analogous to the enzyme and the substrate analogous to the lock.

� Temporary structure called the enzyme-substrate complex formed

� Products have a different shape from the substrate

� Once formed, they are released from the active site

� Leaving it free to become attached to another substrate



Enzyme may be used again

Enzyme-substrate

complex

E

S

P

E

E

P

Reaction coordinate© 2007 Paul Billiet ODWS


� This explains enzyme specificity

� This explains the loss of activity when

enzymes denature


The Induced Fit Hypothesis

� Some proteins can change their shape (conformation)

� When a substrate combines with an enzyme, it induces a change in the enzyme’s conformation

� The active site is then moulded into a precise conformation

� Making the chemical environment suitable for the reaction

� The bonds of the substrate are stretched to make the reaction easier (lowers activation energy)


The Induced Fit Hypothesis

� This explains the enzymes that can react with a

range of substrates of similar types

Hexokinase (a) without (b) with glucose substratehttp://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/ENZYMES/enzyme_mechanism.html


Factors affecting Enzymes

� substrate concentration

� pH

� temperature

� inhibitors


Substrate concentration: Non-enzymic reactions

� The increase in velocity is proportional to the

substrate concentration

Reaction velocity

Substrate concentration


Substrate concentration: Enzymic reactions

� Faster reaction but it reaches a saturation point when all the enzyme molecules are occupied.

� If you alter the concentration of the enzyme then Vmax will change too.

Reaction velocity

Substrate concentration

Vmax


The effect of pHOptimum pH values

Enzyme activity Trypsin

Pepsin

pH

1 3 5 7 9 11


The effect of pH

� Extreme pH levels will produce denaturation

� The structure of the enzyme is changed

� The active site is distorted and the substrate

molecules will no longer fit in it

� At pH values slightly different from the enzyme’s

optimum value, small changes in the charges of the

enzyme and it’s substrate molecules will occur

� This change in ionisation will affect the binding of

the substrate with the active site.


The effect of temperature

� Q10 (the temperature coefficient) = the increase in reaction rate with a 10°C rise in temperature.

� For chemical reactions the Q10 = 2 to 3(the rate of the reaction doubles or triples with every 10°C rise in temperature)

� Enzyme-controlled reactions follow this rule as they are chemical reactions

� BUT at high temperatures proteins denature

� The optimum temperature for an enzyme controlled reaction will be a balance between the Q10 and denaturation.



Temperature / °C

Enzyme activity

0 10 20 30 40 50

Q10 Denaturation



� For most enzymes the optimum temperature is about

30°C

� Many are a lot lower,

cold water fish will die at 30°C because their

enzymes denature

� A few bacteria have enzymes that can withstand very

high temperatures up to 100°C

� Most enzymes however are fully denatured at 70°C


Inhibitors

� Inhibitors are chemicals that reduce the rate of

enzymic reactions.

� The are usually specific and they work at low

concentrations.

� They block the enzyme but they do not

usually destroy it.

� Many drugs and poisons are inhibitors of

enzymes in the nervous system.


The effect of enzyme inhibition

� Irreversible inhibitors: Combine with the

functional groups of the amino acids in the

active site, irreversibly.

Examples: nerve gases and pesticides,

containing organophosphorus, combine with

serine residues in the enzyme acetylcholine

esterase.



� Reversible inhibitors: These can be washed

out of the solution of enzyme by dialysis.

There are two categories.



1. Competitive: These

compete with the

substrate molecules for

the active site.

The inhibitor’s action is

proportional to its

concentration.

Resembles the substrate’s

structure closely.

Enzyme inhibitor complex

Reversible reaction

E + I EI



Succinate Fumarate + 2H++ 2e-

Succinate dehydrogenase

CH2COOH

CH2COOH CHCOOH

CHCOOH

COOH

COOH

CH2

Malonate



2. Non-competitive: These are not influenced by the

concentration of the substrate. It inhibits by binding

irreversibly to the enzyme but not at the active site.

Examples

� Cyanide combines with the Iron in the enzymes

cytochrome oxidase.

� Heavy metals, Ag or Hg, combine with –SH groups.

These can be removed by using a chelating agent such

as EDTA.


Applications of inhibitors

� Negative feedback: end point or end product

inhibition

� Poisons snake bite, plant alkaloids and nerve

gases.

� Medicine antibiotics, sulphonamides,

sedatives and stimulants


enzymeslibvolume7.xyz/.../enzymes/enzymespresentation1.pdf · · 2015-01-06enzyme controlled...

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