ENZYME

Characteristic Mechanism Factors

Definition

Enzymes are protein produced by the cells of living organism act as biological catalyst by controlling and accelerating the rate of biochemical reactions in the cell at fairly low temperatures.

Without enzymes, biochemical reactions become too slow to maintain the life supporting process in an organism.

EA

with enzyme

Figure 5.5B

Reactants

Products

EA

without

enzyme

Net change in energy

Important Terms…

The activation energy, EA

Is the initial amount of energy needed to start a chemical reaction

Is often supplied in the form of heat from the surroundings in a system

Cont…

The active site Is the region on the enzyme where the

substrate binds

Figure 8.16

Substate

Active site

Enzyme

(a)

Cont…

Cofactors Are nonprotein enzyme helpers

Coenzymes Are organic cofactors

Enzyme Inhibitors Competitive inhibitors

Bind to the active site of an enzyme, competing with the substrate

(b) Competitive inhibition

A competitiveinhibitor mimics the

substrate, competingfor the active site.

Competitiveinhibitor

A substrate canbind normally to the

active site of anenzyme.

Substrate

Active site

Enzyme

(a) Normal binding

Cont…

Noncompetitive inhibitors Bind to another part of an enzyme,

changing the function

A noncompetitiveinhibitor binds to the

enzyme away fromthe active site, altering

the conformation ofthe enzyme so that its

active site no longerfunctions.

Noncompetitive inhibitor

(c) Noncompetitive inhibition

Competitive and noncompetitive inhibition

Types of enzymes

1. Intracellular enzyme Enzymes that are produces in certain cells

and remain to react in the cell These enzymes could exist either in

cytoplasm (in organelles) or nucleus.

2. Extracellular enzyme Some enzymes are produced by cell but then

transported out of the cell for action outside the cell

Enzyme(sucrase)

Activesite

1

2

3

Substrate(sucrose)

Enzyme available with empty active site

Substrate binds to

enzyme with induced fit

Substrate is converted to

products

4

Products are released

Glucose Fructose

How an enzyme works

The enzyme is unchanged and can repeat the process

Figure 5.6

Characteristics

1. Enzymes generally act quickly the speed of reaction is usually stated in ‘turnover

number’ (refer to the number of substrates)

2. Enzymes are not damaged however, this does not mean that enzymes can be

used repeatedly forever without replacement

3. Enzymes can react in both direction

4. Enzymes are specific each enzymes limited to one specific reaction that

involved one specific substrate only

Cont…5. An enzymes molecule is usually bigger

than its substrates6. All enzymes are proteins and not all

protein are enzymes7. Enzymes are complex globular protein

and three dimensional.

Main function of enzymes Increase the rate of chemical reaction by

lowering activation energy

Mechanisms of EnzymesThere are 2 main hypothesis explaining the mechanism of enzyme action : The lock-and-key hypothesis The induced-fit hypothesis

Simple mechanism

Lock-and-key hypothesis

The hypothesis proposed that the active site and substrate are exactly complementary

An enzyme is a large globular protein with specific three dimensional shape.

It has a site called the active site. In the lock-and-key hypothesis, the shape of the substrate (‘key’) fits

into the active site of the enzyme (‘lock’) forming and enzyme-substrate complex

Reaction takes place and products are formed and released

Induced-fit hypothesis

It is a modified version of the lock-and-key hypothesis

The hypothesis suggested that active site is flexible and is not exactly complementary to the shape of the substrate

An enzyme collides with the substrate molecule. The substrate binds to the active site

The binding induces a slight change in the shape of the enzyme to enclose the substrate making the fit more precise.

factors

Any factors that affect the activity of an enzyme will change the rate of the reaction catalysed by that enzyme

Enzyme characteristic are affected by several factors such as temperature, pH, substrate and enzyme concentration.

The effect of Temperature At lower temperature, enzymes

are not active As the temperature rises, the

substrate and the enzyme molecules move rapidly and are more likely to collide

The increase in temperature to a specific level can increase the rate of the enzyme activity until the optimum temperature rate.

After the optimum temperature the enzymes activities decreased and totally stopped at temperature of ±60oC

The effect of pH

All enzymes have a specific optimum pH at which they function most efficiently

Most enzyme act at a pH in range of 5 – 9, and reactions most efficiently at a pH of 7

The small change in pH can produce a large effect on enzyme activity

A pH that is extreme usually destroy enzymes

Cont…

However, there are exceptions for certain enzymes such as:

Pepsin pH 1.5 – 2.5 (acidic) Rennin pH 8.5 (alkaline)

The effect of Enzyme Concentration

The rate of an enzyme-catalysed reaction is directly proportional to the concentration of the enzymes if substrates are present in excess concentration and no other factors are limiting

The effect of Substrate Concentration

Increasing the substrate concentration can give a increasing in reaction rate

At lower concentrations the rate increase in direct proportion to the substrate concentration

At higher substance

concentration the rate of

reaction becomes

constant

classification

Enzymes can be classified by the kind of chemical reaction catalyzed:

1. Hydrolases

2. Lyases

3. Isomerase

4. Ligases

5. Transferases

6. Oxidoreductase

This classification system introduced in 1961 by IUB (International Union of Biochemistry)