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ENZYMESA protein with catalytic properties due to its

power of specific activation

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Chemical reactions Chemical reactions need an initial input of

energy = THE ACTIVATION ENERGY

During this part of the reaction the moleculesare said to be in a transition state.

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Reaction pathway

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Making reactions go faster Increasing the temperature make molecules move

faster

Biological systems are very sensitive to temperaturechanges.

Enzymes can increase the rate of reactions without

increasing the temperature.

They do this by lowering the activation energy.

They create a new reaction pathwaya short cut

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An enzyme controlled pathway

Enzyme controlled reactions proceed 10 exp 8 to 10 exp 11

times faster than corresponding non-enzymatic reactions.5

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Enzyme structure Enzymes are

proteins

They have aglobularshape

A complex 3-Dstructure

Human pancreatic amylase

Dr. Anjuman Begum

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http://www.chem.ubc.ca/personnel/faculty/withers/group/group/begum/begum.htmhttp://www.chem.ubc.ca/personnel/faculty/withers/group/group/begum/begum.htmhttp://www.chem.ubc.ca/personnel/faculty/withers/group/group/begum/begum.htmhttp://www.chem.ubc.ca/personnel/faculty/withers/group/group/begum/begum.htm

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The active site One part of an enzyme,

the active site, is

particularly important

The shapeand the

chemical environment

inside the active site

permits a chemicalreaction to proceed

more easily

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Cofactors An additional non-

protein molecule that isneeded by someenzymes to help the

reaction Tightly bound cofactors

are called prostheticgroups

Cofactors that are bound

and released easily arecalled coenzymes

Many vitamins arecoenzymes Nitrogenase enzyme with Fe, Mo and ADP cofactors

Jmol from a RCSB PDB file 2007 Steve CookH.SCHINDELIN, C.KISKER, J.L.SCHLESSMAN, J.B.HOWARD, D.C.REES

STRUCTURE OF ADP X ALF4(-)-STABILIZED NITROGENASE COMPLEX AND ITS

IMPLICATIONS FOR SIGNAL TRANSDUCTION; NATURE 387:370 (1997)8
http://www.steve.gb.com/science/nitrogen_metabolism.htmlhttp://www.steve.gb.com/science/nitrogen_metabolism.htmlhttp://www.steve.gb.com/science/nitrogen_metabolism/nitrogenase.html

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The substrate The substrate of an enzyme are the reactants

that are activated by the enzyme

Enzymes are specificto their substrates The specificity is determined by the active

site

2007 Paul Billiet ODWS 9
http://www.saburchill.com/IBbiology/bio_hp.htmlhttp://www.saburchill.com/IBbiology/bio_hp.html

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The Lock and Key Hypothesis

Fit between the substrate and the active site of the enzyme isexact

Like a key fits into a lock very precisely

The key is analogous to the enzyme and the substrateanalogous to the lock.

Temporary structure called the enzyme-substrate complexformed

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

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Enzyme may

be used againEnzyme-

substratecomplex

E

S

P

E

E

P

Reaction coordinate11

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This explains enzyme specificity

This explains the loss of activity when

enzymes denature

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The Induced Fit Hypothesis

Some proteins can change their shape(conformation)

When a substrate combines with an enzyme, it

induces a change in the enzymes conformation The active site is then moulded into a precise

conformation

Making the chemical environment suitable for thereaction

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

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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

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http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/ENZYMES/enzyme_mechanism.htmlhttp://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/ENZYMES/enzyme_mechanism.html

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Factors affecting Enzymes

substrate concentration

pH

temperature

inhibitors

2007 Paul Billiet ODWS15

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Substrate concentration: Non-enzymic reactions

The increase in velocity is proportional to the

substrate concentration

Reaction

velocity

Substrate concentration

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Substrate concentration: Enzymic reactions

Faster reaction but it reaches a saturation point when all theenzyme molecules are occupied.

If you alter the concentration of the enzymethen Vmaxwillchange too.

Reaction

velocity

Substrate concentration

Vmax

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The effect of pHOptimum pH values

Enzyme

activity Trypsin

Pepsin

pH

1 3 5 7 9 11

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The effect of pH

Extreme pH levels will produce denaturation

The structure of the enzyme is changed

The active site is distorted and the substratemolecules will no longer fit in it

At pH values slightly different from the enzymes

optimum value, small changes in the charges of the

enzyme and its substrate molecules will occur

This change in ionisation will affect the binding of

the substrate with the active site.

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The effect of temperature

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

For chemical reactions the Q10 = 2 to 3

(the rate of the reaction doubles or triples with every10C rise in temperature)

Enzyme-controlled reactions follow this rule as theyare chemical reactions

BUT at high temperatures proteins denature The optimum temperature for an enzyme controlled

reaction will be a balance between the Q10 anddenaturation.

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Temperature / C

Enzymeactivity

0 10 20 30 40 50

Q10 Denaturation

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For most enzymes the optimum temperature is about

30C

Many are a lot lower,

cold water fish will die at 30C because their

enzymes denature

A few bacteria have enzymes that can withstand very

high temperatures up to 100C Most enzymes however are fully denatured at 70C

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Inhibitors

Inhibitors are chemicals that reduce the rate of

enzymic reactions.

The are usually specific and they work at lowconcentrations.

They block the enzyme but they do not

usually destroy it. Many drugs and poisons are inhibitors of

enzymes in the nervous system.23

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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 acetylcholineesterase.

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Reversible inhibitors:These can be washed

out of the solution of enzyme by dialysis.

There are two categories.

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1. Competitive: These

compete with the

substrate molecules for

the active site.

The inhibitors action is

proportional to its

concentration.Resembles the substrates

structure closely.

Enzyme inhibitor

complexReversible

reaction

E + I EI

2007 Paul Billiet ODWS26

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Succinate Fumarate + 2H++ 2e-

Succinate dehydrogenase

CH2COOH

CH2COOH CHCOOH

CHCOOH

COOH

COOH

CH2

Malonate

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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, Agor Hg, combine with

SHgroups.

These can be removed by using a chelating agent such

as EDTA.

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Applications of inhibitors

Negative feedback: end point or end product

inhibition

Poisonssnake bite, plant alkaloids and nervegases.

Medicineantibiotics, sulphonamides,

sedatives and stimulants

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