Biochemistry (Lippincott's Illustrated Reviews Series), 6E

Enzymes

OMAR A. ALOMAIRBiochemistry 1

ReferencesBiochemistry (Lippincott's Illustrated Reviews Series), 6E

Enzymes

• Almost every reaction that happen in the body involves enzymes

• They function as a catalyst from physiological reaction

• Enzymes control in a selective manner reactants in chemical

reactions

• These reactants are called substrate

• Consequently, Enzymes mediate all metabolic interactions in the

body

EnzymesNomenclature

• Enzymes are usually named in two ways

• The first one is the “Recommended naming system”

• This naming scheme is more appropriate and easier for every day use

• The second is the “Systematic naming system”

• This naming scheme is more complicated and is required to accurately

describe an enzyme

EnzymesNomenclature “Recommended naming system”• The commonly encountered enzymes are giving the suffix “ase”

• The “ase” suffix is linked the substrate of the reaction, e.g.

glucosidase and urease

• Or it is attached to the characteristic of the chemical reaction, e.g.

lactate dehydrogenase and adenylyl cyclase

• However, a number of enzymes are usually named arbitrarily

without a description of their functions, e.g. trypsin and pepsin

EnzymesNomenclature “Systematic naming system”• According to this nomenclature, all enzymes are classified under 6

classes

• For any enzyme, the suffix “ase” is part of the chemical

modification initialed by the enzyme

• Additionally, all the involved substrates written a head of the

chemical reaction, e.g. lactate :NAD+ oxidoreductase

• Although this naming scheme is very informative, it can be a bit

confusing and unwieldy

EnzymesNomenclature “Systematic naming system”

EnzymesNomenclature “Systematic naming system”

EnzymesProperties: 1. Active site specificity• Each enzyme has a unique empty space within its structure

named the active site

• The active site contains amino acids portion that participate in the

substrate interactions to form enzyme–substrate (ES) complex

• This binding catalyzes the substrate via structural transformation

EnzymesProperties: 1. Active site

EnzymesProperties: 2. Catalytic efficiency• Efficiency is one of the most important properties of enzymes

•When enzymes are not catalyzing a reaction, it takes 108 longer

• The amount of byproduct that results from the catalytic reaction is

called the turnover number

• This number range between 102-104 product per second

EnzymesProperties: 3. Coenzymes and Cofactors• Some enzymes can not facilitate a catalytic reaction with a

substrate without the presence of certain molecules

•When these molecules composed of metallic ions such as Zn2+ or

Fe2+, it is called cofactors

• On the other hand, if the extra molecules organic in nature it is

denoted as coenzymes. E.g. vitamins B1

EnzymesProperties: 3. Location• Enzymatic reactions take place in specific location within a cell

• This aspect of enzyme compartmentalization provide the specify

of enzymes

• Since there are thousands of enzymes within a cell, isolating

enzymes from their substrates into specific organelles helps in

organizing the huge number of pathways

EnzymesProperties: 3. Location

EnzymesFactors affecting rate of reaction1. Substrate concentration• The velocity of the rate of reaction (V) is equal to “the number of

substrate molecules converted to product per unit of time”

• The of products produced keep forming is proportional to the

substrate concentration

• This increase in the rate enzymatic reaction is limited by the

maximum velocity of the reaction and not the substrate

concentration

EnzymesFactors affecting rate of reaction1. Substrate concentration

Enzyme 1

Enzyme 2

EnzymesFactors affecting rate of reaction2. Temperature• Until a maximum velocity is reached, the velocity of enzymatic

reaction is increased as the temperature increases

• This is due to the increase the amount of substrate complex

having sufficient energy

• Any further increase in the temperature will retard the rate of the

reaction

• The reason behind this is the denaturation of the enzyme

EnzymesFactors affecting the rate of reaction1. Temperature

EnzymesFactors affecting the rate of reaction3. pH• Reaction velocity is controlled by the concentration of (H+)

• The catalytic activity of enzyme require the targeted chemical

group to be in a specific ionized form

• Similar to high temperature, extreme pH value can slow the rate

of reaction due to the degradation of the enzyme itself

• Each enzyme has preferred range of pH within the body

EnzymesFactors affecting the rate of reaction3. pH

EnzymesInhibition• Inhibitors are any substance that interfere with the enzymatic

reaction

• These inhibitors can be irreversible when it form a covalent bond,

e.g. Lead binds covalently with ferrochelatase preventing heme

synthesis

• They can also be reversible when covalent binding is not a factor

EnzymesReversible Inhibition• Reversible inhibition can be classified to either competitive or

non-competitive inhibition

• Competitive inhibitors occupy the site of the substrate

• Non-competitive inhibitors bind to a different site form the

substrate

EnzymesReversible Inhibition

Competitive inhibitionNon-Competitive

inhibition

EnzymesClinical Applications• Different enzymes are released from different part of the body

• The level of the enzyme in circulatory system can be correlated to

a number of disease

• Enzyme level in serum can be quantified and compared to normal

levels

• For example, the enzyme alanine aminotransferase (ALT) presence

at high level in blood is an indication of liver disease

EnzymesClinical Applications

Enzyme level in blood