Enzymes

Enzymes Compiled & Edited by

Dr. Syed Ismail

VN Marathwada Agril. University,

Parbhani, India

Compiled & Edited by Dr Syed Ismail, MKV Parbhani 1

http://www.stark.kent.edu/~cearley/PChem/protein/protein.htm

Enzymes are complex chemicals that

control reactions in living cells. They are

biochemical catalysts speeding up

reactions that would otherwise happen too

slowly.



• Biological catalysts – a molecule which speeds up a chemical reaction but remains unchanged at the end of the reaction

• Generally names end in –ase

• Globular proteins (soluble)

• Active site – region (cleft or depression) to which another molecule or molecules (substrate) can bind

• Substrate fit perfectly and is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme’s amino acids (enzyme-substrate complex)

• Specific - shape of active site will only allow one shape of molecule to fit


The Chemical nature of enzymes

Enzymes are globular proteins. They have a complex tertiary and quaternary

structure in which polypeptides are folded around each other to form a roughly

spherical or globular shape. The overall 3D shape of an enzyme molecule is very

important: if it is altered, the enzyme cannot bind to its substrate and so cannot

function. Enzyme shape is maintained by hydrogen bonds and ionic forces.

Enzymes have several important properties:

Enzymes are specific: each enzyme usually catalyses only one reaction.

Enzymes combine with their substrates to form temporary enzyme-substrate

complex.

Enzymes are not altered or used up by the reactions they catalyze, so can be

used again and again.

Enzymes are sensitive to temperature and pH.

Many enzymes need cofactors in order to function.

Enzyme function may be slowed down or stopped by inhibitors.

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ENZYME SUBTRATE or REACTION TYPE

Maltase Maltose

Urease Urea

Proteases Proteins

Carbohydrases Carbohydrates

Lipases Lipids

Hydrolases Hydrolysis Reaction

Deaminases Removing amines

Dehydrogenases Removing hydrogens

NOMENCLATURE


9

OXIDOREDUCTASES

CATALYZE REDOX REACTIONS

SOME COMMON TYPES:

– REDUCTASES

– DEHYDROGENASES

– OXIDASES

– OXYGENASES

LOOK FOR INVOLVEMENT OF COENZYMES

– NAD+ , NADP+, FAD, FMN

10

TRANSFERASES

TRANSFER OF FUNCTIONAL GROUPS

SOME EXAMPLES:

– TRANSFERASES

– PHOSPHORYLASES

TRANSFER OF Pi OR PPi

– KINASES

A PHOSPHATE TRANSFER

UTILIZE OR GENERATE ATP

11

HYDROLASES

HYDROLYSIS REACTIONS

WATER MOLECULES USED TO BREAK BONDS

EXAMPLES:

– PHOSPHATASES

– PEPTIDASES

– ATPase, GTPase

12

LYASES

CATALYZES REACTIONS THAT

– GENERATE A DOUBLE BOND

– ADDS A SUBSTRATE MOLECULE TO DOUBLE BOND OF A SECOND SUBSTRATE

EXAMPLES:

– DECARBOXYLASES

– DEHYDRATASES

– ALDOLASE

13

ISOMERASES

CONVERSION OF ONE ISOMERIC FORM

INTO ANOTHER

EXAMPLES:

– ISOMERASES

– EPIMERASES

– RACEMASES

– MUTASES

14

LIGASES

TWO MOLECULES ARE JOINED

ANABOLIC REACTIONS

REQUIRE NUCLEOTIDES (ATP, GTP) TO DRIVE THEM – A PYROPHOSPHATE BOND MUST BE BROKEN

EXAMPLES: – CARBOXYLASE

– SYNTHETASES


Activation energy

• Enzymes increase the rate at which chemical reactions occur

• Activation energy – extra energy temporarily given to substrate to convert it to product


Like all catalysts, enzymes work by lowering the activation

energy for a reaction, thus dramatically accelerating the rate of

the reaction.

Most importantly;Enzymes are not used up by the reaction. After

they have done their work they release the products and are not

changed.



18

How do enzymes Work?

Enzymes work by

weakening bonds which

lowers activation

energy


20

• Vmax occurs when

enzyme active sites are

saturated with substrate

• Km (Michaelis-Menten

constant) reflects affinity

of enzyme for its

substrate

• smaller the Km, the

greater the affinity an

enzyme has for its

substrate




















FACTORS AFFECTING ENZYME ACTIVITY

Following are the important factors

affecting the enzyme activity.

Temperature

pH

Salt & Inorganic ions

Enzyme concentration

Inhibitors

Cofactors and coenzymes











Inhibitors Inhibitors slow down or stop enzyme reaction. Usually, enzyme

inhibition is a natural process, a means of switching enzymes on or off

when necessary.

Inhibition can be reversible and the enzyme returns to full activity once

the inhibitor is removed. Drugs and poisons can inhibit particular

enzymes, this type of inhibition is often non-reversible.

Reversible inhibitors are either competitive or non-competitive.

Competitive inhibitors

Compete with normal substrate molecules to occupy the active

site. A competitive inhibitor fits into the active site of the enzyme

preventing the real substrate from gaining access. The inhibitor cannot be

converted to the products of the reaction and so the overall rate of

reaction is slowed down.


Competitive Inhibition

This is where the inhibitor is a molecule which has a similar shape to the molecule which is supposed to be binding to the active site. In the case of enzymes, a competitive inhibitor may have the same shape as that of the substrate, but it doesn't react in the same way. Rather than turning into the product, it simply uses up time and prevents the substrate from getting to the active site. It blocks the way.

ALBIO9700/2006JK


Non-competitive Inhibition

• This is the kind that exists when a molecule binds to a different site on the enzyme, rendering it inactive. Sometimes it does this before the substrate reaches the active site, sometimes afterwards, but in either case it stops the enzyme doing its job, and prevents a product being formed.


• In both competitive and non-competitive inhibition, • it is possible to have both reversible and irreversible

inhibitors.

• As the name suggests, a reversible inhibitor does not have a permanent affect - it will stop the enzyme doing what it is supposed to do,

• but it will move off again and allow the enzyme to function later on;

• an irreversible inhibitor, on the other hand, permanently renders the enzyme inactive, so it will have to be replaced by a brand new one .

ALBIO9700/2006JK


End-product inhibition


54

Cofactors and Coenzymes

• Inorganic substances (zinc, iron) and vitamins

(respectively) are sometimes need for proper

enzymatic activity.

• Example:

Iron must be present in the quaternary structure -

hemoglobin in order for it to pick up oxygen.



Coenzymes

are not proteins and so are not inactivated by

heat. Examples of coenzymes are the vitamins or

compounds derived from vitamins. The reaction

involving a coenzyme can be written as follows:

coenzyme + apoenzyme = enzyme

Coenzyme A is essential in the metabolism of

carbohydrates, lipids, and proteins in the body.

APOENZYMES and

COENZYMES


•Coenzymes are bound at the active site in order to interact with the substrate and play an essential role in the catalysed reaction.

•They act as carriers of a variety of chemical groups.


Most water-soluble vitamins are components of coenzymes

Vitamin Coenzyme Deficiency

Thiamine (B1) Thiamine

pyrophosphate Beriberi (weight

loss,other problems

Riboflavin (B2) FAD+ Mouth lesions, dermatitis

Nicotinic acid (niacine)

NAD+ Pellagra (dermatitis,

depression)

Pantohtinic acid Coenzyme A Hypertension

Biotin Biotin Rash, muscle pain


Zymogen

Pepsinogen

Chymotrypsinogen

Trypsinogen

Procarboxypeptidase

Proelastase

Prothrombin

Fibrinogen

Factor VII

Factor X

Proinsulin

Procollagen

Procollagenase

Active Enzyme

Pepsin

Chymotrypsin

Trypsin

Carboxypeptidase

Elastase

Thrombin

Fibrin

Factor VIIa

Factor Xa

Insulin

Collagen

Collagenase

Function

protein digestion

protein digestion

protein digestion

protein digestion

protein digestion

blood clot formation




plasma glucose

homeostasis

component of skin and

bone remodeling

processes during

metamorphosis, etc.


http://www.stark.kent.edu/~cearley/PChem/protein/protein.htm

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Enzymes

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