Tishk International University

Faculty of Science

Department of Medical Analysis

General and systemic Biochemistry

Enzyme

Ass. Prof. Dr. Rundk A. Hwaiz

Grade 2-Fall 2021-2022

Dr. Rundk A. Hwaiz

Introduction

Enzymes are biological catalyst that accelerate the rate of chemical reacton without

being consumed or produced in the reaction

Enzymes [E ], at the first step react with substrate [S] produce complex [ES],

the second step [ES] complex convert to product [P], and [E]. So the substrate

convert to product, and the enzyme remain as the same without consume or any

change in structure, cause increase rate of chemical reaction and decrease activation

energy of the reaction.

E + S ES E +P

Properties of enzymes

• Nature: organic and mostly protein in nature. It could be RNA which called

ribosomes that act on phospho diester bond in nucleotides.

• Specifity: enzymes are specific.

• Production: enzymes produces inside the living cells but can act in and out

living cells.

• Mode of action: the enzymes function is to accelerate the rate of chemical

reaction by decreasing the energy of activation.

• Feedback: enzyme activity can be increased or decreased according the body

needs.

Proteins nature

Protein could be simple (only amino acids) or complex (there is another group

rather than protein or amino acids.

Simple enzyme like lactase which only contain amino acids.

Complex enzyme contain protein part and non protein part, all together called

Holo-enzyme. In holo enzyme the protein part called apo-enzyme, and non protein

part called co-factor. Co-factor is co-enzyme (organic substance) or metal (inorganic

substance) like Mn and Mg.

Usually the organic co-enzymes are loosly attached and usually are vitamins, and

they called Metal activated enzyme.

But if the metal is tightly attached or integrated in the formation of enzyme, and is

part of enzyme, so they called Metalo enzyme.

Functions of co-factor are:

• Activation of enzyme

• Carrier

• Doner

Terminology

Old name like pepsin

Substrate + ase like lactase

Substrate + action like lactate dehydrogenase.

Mode of action

1. Energy change:

Enzymes react with substrate to give product and accelertate the reaction

by lowering energy of activation.

2. Active site or catalytic site:

• Lock and key (fischer hypothesis): the active site is rigid shape must suits the

substrate.

• Induced fit (Koshland): the active site is not rigid nor pre-shaped, but will

change the shape of activesite to be suitable to bind to substrate.

In fischer, the enzyme shows absolute specificity, it act only on one substrate.

But in koshland, it act on several of substrate so it shows less specifity.

Enzyme specifity

1. Absolute, only one sub like argenase act on arginine.

2. Dual, act on two substrate like xanthine oxidase act on xanthine and

hypoxanthine.

3. Relative, specific to certain type of bond like lipase act on ester bond.

4. Group realtive, act on the specific bond formed by specific group, like pepsin

which act on peptide bond formed between aromatic AA or trypsin act on

peptide bonds with basic AA.

Factors affecting the enzyme and reaction velocity

Temperature

pH

Enzyme concentration

Substrate concentration

Temperature

The optimum temperature of an enzyme is that temp. at which the greatest amount

of substrate is changed in unit time . For example in the case of G.I.T. enzymes,

the optimum temp. is around 40 C.

Generally the velocity of an enzyme catalyzed reaction increases as temp. rises.

pH

Each enzyme has an optimum pH at which the enzyme reacts at maximum speed or

has the greatest activity.

Generally moderate pH changes affect the ionic states of enzyme and substrate .

Most enzymes are inactivated in strong acid or alkaline solutions, for example

pepsin loses its activity rapidly above pH 8 and sucrase below pH 3 because

enzymes are proteins and they denaturate at high acid or alkaline media.

Shortly the optimum pH of most enzymes

is between pH5 and pH 9.

Enzyme concentration

Within wide limits the speed of an enzymatic reaction is proportional to the

enzyme concentration when the substrate concentration is maintained

constant, as its illustrated by the following figure :

V

[En.]

0.001 0.002 0.003 0.004

8

6

4

2

12

10

Substrate concentration [ S ]

If the concentration of the substrate [s] is increased while all other conditions are

kept constant, the velocity of an enzyme catalysed reaction increases to maximum

value and then no further increase, since the enzyme will be saturated with substrate.

i

i

E + S ES E +PK1

K-1

K2

Km=K-1+K2/K1

To measure the affinity between enzyme and substrate, the lower the Km the

higher the affinity between enzyme and substrate.

Enzyme inhibitors or enzyme inhibition

Any substance that can diminish the velocity of an enzyme-catalyzed reaction is

called an inhibitor.

The pharmacological action of many drugs depend largely on enzyme

inhibition.

The inhibitor resembles closely in structure to substrate so it can combines

reversibly with the enzyme forming an EnI complex instead of EnS complex which

is normally formed. When both the substrate and this type of inhibitor are present,

they compete for the same binding site on the enzyme surface.

The family of drugs known as Angiotensin Converting Enzyme ( ACE ) inhibitors

are a good example of enzyme inhibitors as drugs .

ACE

Angiotensin I Angiotensin II

S ; inactive ( decapeptide)

P; potent pressure

Capotril (decapeptide) = I

Statins

Statins are group of drugs such as Atorvastatin, they inhibit production of

cholesterol, take by patients with hypercholesterolemia, this drug inhibit the enzyme

3hydroxy, 3methyl glutaryl CoA reductase (HMG CoA reductase), this enzyme

catalyze convert HMG CoA to mevalonic acid, a rate-limiting step for cholesterol

production, so cause decrease cholesterol in blood.

CLASSIFICATION OF ENZYMES

1-Oxido-reductases

2-Transferases

3-Hydrolases

4-Lyases

5-Isomerases

6-Ligases or Synthetases

1-Oxidoreductases:

These are enzymes catalyse oxido-reduction reactions between substrates.

2H2O2 2H2O + O2 Catalase

2-Transferases:

These are enzymes catalysing reactions involving transfer of group from

molecule to other molecule, such as creatine kinase, glucokinase and others.

Creatine +ATP Creatine phosphate + ADPKinase

3-Hydrolases:

These are enzymes catalysing reactions involving hydrolysis, such as maltase,

sucrase, protease.

Maltose + H2O Glucose + GlucoseMaltase

4-Lyases:

These are enzymes catalyse the removal of atoms or groups from substrates by

mechanisms other than hydrolysis leaving double bond (s), such as fumarase.

Fumarate + H2O MalateFumarase

5-Isomerases:

These are enzymes catalyzing interconversion of optical, positional and

geometrical isomers, for example Phospho-hexoseisomerase.

G—6—P. F—6—P. Phospho-hexoseisomerase

6-Ligases or Synthetases :

These are enzymes catalysing the linking together of two elements or compounds,

depending on the chemical nature of the bond which is formed, example citrate

synthase.

Oxaloacetate + Acetyl CoA Citrate

NH3 + CO2 + 2ATP Carbamoyl phosphate+2ADP +Pi

citrate synthase

Carbamoyl phosphate synthetase

Activation of enzymes: Activation of enzyme can occur via co-factor but also

as below:

ZYMOGENS: Are ( pro-enzymes ) : these are the inactive precursors of enzymes,

examples:

Zymogens Active form of enzymes

Pepsinogen activate by HCL in stomach to Pepsin

Trypsinogen activate by enterokinase to Trypsin

Prothrombin activate by factor X to Thrombin