CHMI 2227 - E.R. Gauthier, Ph.D. 1 CHMI 2227E Biochemistry I Enzymes: - Basic concepts

CHMI 2227 - E.R. Gauthier, Ph.D. 1

CHMI 2227EBiochemistry I

Enzymes:- Basic concepts


Enzymes Enzymes:

Proteins RNA

Catalysts: Fully active enzyme is regenerated at the end of the reaction

Speed-up chemical reactions in cells by placing the substrate in an environment that facilitates the reaction

So: proper 3D structure (tertiary and, when applicable, quaternary) is ABSOLUTELY ESSENTIAL for the function of enzymes;


Enzymes

Very powerful catalysts;

Very specific for their substrate; Can distinguish between

enantiomers

Some enzymes require additional chemicals or groups to function: Metal ions Prosthetic groups:

Heme Co-factors

ATP


Classes of enzymes

1. oxido-reduction: oxidoreductases

2. Transfer of chemical groups: transferases

3. Hydrolysis: hydrolases

4. Removal of chemical groups: lyases

5. Isomerisation: isomerases 6.Linking two groups together:

ligases

Only 6 types of chemical reactions are catalyzed by enzymes:

Enzymes are classified according to the type of reaction they catalyse: each enzyme is given a systematic name and a IUBMB number (EC XXXX)

IUMBM = International Union of Biochemistry and Molecular Biology

The complete list of enzymes can be found at the following website: www.chem.qmul.ac.uk/iubmb/enzyme/


IUBMB enzyme nomenclature

ATP + D-glucose ADP + D-glucose-6-phosphate

Example: ATP: D-glucose-6 phosphotransferase (aka hexokinase)

EC 2.7.1.1 2 = transferase 7 = phosphotransferase 1 = acceptor group is OH 1 = glucose binds the

phosphate group


Classes of enzymes

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


Classes of enzymes 1- Oxidoreductases

2- Transferases

3- Hydrolases

4- Lyases

5- Isomerases

6- Ligases

A- + B A + B-

A-B + C A + B-C

A-B + H2O A-H + B-OH

A=B + X-YA-BX Y

A + B A-B

A-BX Y

A-BY X


1. OxidoreductasesExample: Lactate dehydrogenase (EC 1.1.1.27)

Lactate = substrate Pyruvate = product NAD+/NADH = co-factor


2. TransferasesExample: Alanine transaminase (EC 2.6.1.2)

Alanine / -ketoglutarate = substrates Pyruvate / L-glutamate = products


3. HydrolasesExample: diphosphate phosphohydrolase(EC 3.6.1.1)

Pyrophosphate / H2O = substrates Phosphate = product


4. Lyases (aka synthases)Example: Pyruvate decarboxylase(EC 4.1.1.1)

Pyruvate = substrate Acetaldehyde and CO2 = products


5. IsomerasesExample: Alanine racemase(EC 5.1.1.1)

L-alanine = substrate D-alanine = product


6. Ligases (aka synthetases)Example: L-glutamine synthetase(EC 6.3.1.2)

L-glutamate / NH4+ = substrates

L-glutamine = product ATP = co-factor


How do enzymes work?

Enzymatic reactions take place in multiple steps involving reaction intermediates:

E + S ES EP E + P

Note that, while in theory these reactions are reversible, in practice, the low levels of the one of the reactants (S or P) usually pushes the equilibrium in one direction;

Enzymes increase the rate of chemical reactions, but do NOT alter the direction of the equilibrium.


Enzymes and the energy barrier In chemical reactions, three conditions

must be met for a reaction to take place:

1. the molecules must collide to react. If two molecules simply collide, however, they will not always react; therefore, the occurrence of a collision is not enough.

2. there must be enough energy (energy of activation) for the two molecules to react. If two slow molecules collide, they might bounce off one another because they do not contain enough energy to reach the energy of activation and overcome the transition state (the highest energy point).

3. the molecules must be orientated with respect to each other correctly.


Enzymes and the energy barrier

Gŧ

G

The transition state is not a reaction intermediate: it is a transitory molecular structure that is no longer the substrate, but not yet the product.

G = Gibbs free energy the ability of the molecule to react The greater the free energy, the more

unstable the molecule is.

G = Gproduct – Gsubstrate: If G > 0 = reaction does not occur

spontaneously (because S is more stable that P)

If G < 0 = reaction occurs spontaneously (because P is more stable than S)

In other words: the more negative the G, the more likely the reaction will take place;


Enzymes and the energy barrier

In the absence of an enzyme (or catalyst), the substrate (here the metal stick) requires a substantial amount of energy in order to reach the activation state and react;


Enzymes and the energy barrier In the presence of an enzyme, the reaction is facilitated because the

enzyme provides a better environment for the reaction to occur: Close proximity of substrate and chemical groups of the enzyme Proper orientation of the chemical groups with respect to the substrate The formation of the transition state is favoured;

This results in a lowering of the activation energy required for the reaction to occur.


The « lock and key » myth…

The « lock and key » thing is a MYTH: If the enzyme and substrate were perfectly

complementary, like a lock and key, the interaction between E and S would be so stable that the reaction would not occur!


The « lock and key » myth… Instead, the 3D shape of the enzyme is complementary to the

transition state;

by doing so, the enzymes favours the formation of the transition state, lowers the energy of activation, and accelerates the reaction…COOL!

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CHMI 2227 - E.R. Gauthier, Ph.D. 1 CHMI 2227E Biochemistry I Enzymes: - Basic concepts