Enzyme Activity Essay

Enzyme

‫السلام عليكم ورحمة ا وبركاته‬

Why Are Enzymes So Important?
Why are we devoting two whole lecture topic to a enzyme?
Nearly all chemical reactions in biological cells need enzymes to make the reaction occur fast enough to support life.
From the Virtual Cell Biology Classroom on ScienceProfOnline.com

Image: Jumping rope, Meagan E.

Outline
• Composition, structure and properties of enzyme
• How Enzymes work
• Enzyme activity
• Factors affecting enzyme activity • Regulation of enzyme activities
• Enzymes in clinical diagnosis

1. Definition of enzyme
•Enzymes are biological catalysts.
•A Catalyst is defined as "a substance that increases the rate of a chemical reaction without being itself changed in the process.”

Enzymes as Biological Catalysts
• Enzymes are proteins that increase the rate of reaction by lowering the energy of activation
• They catalyze nearly all the chemical reactions taking place in the cells of the body • Enzymes have unique threedimensional shapes that fit the shapes of reactants
(substrates)

2. Properties of enzymes (important!)
• Catalytic efficiency – high efficiency, 103 to 1017 faster than the corresponding uncatalyzed reactions
• Specificity - high specificity, interacting with one or a few specific substrates and catalyzing only one type of chemical reaction. • Mild reaction conditions37℃, physiological pH, ambient atmospheric pressure 3. Chemical composition of enzymes
(1) Simple protein
(2) Conjugated protein
Holoenzyme= Apoenzyme+
Cofactor

Cofactor

Coenzyme : loosely bound to enzyme
(non-covalently bound).
Prosthetic group : very tightly or even covalently bound to enzyme (covalently bound) 4. Classification of enzymes
(1). By their composition
1). Monomeric enzyme
2). Oligomeric enzyme
3). Multienzyme complex: such as Fatty acid synthase

(2) Nomenclature
• Recommended name
•Enzymes
are usually named according to the reaction they carry out. •To generate the name of an enzyme, the suffix -ase is added to the name of its substrate (e.g., lactase is the enzyme that cleaves lactose) or the type of reaction
(e.g., DNA polymerase forms DNA polymers). •Systematic name (International

5. How enzymes work (important!)
1) Enzymes lower a reaction’s activation energy
– All chemical reactions have an energy barrier, called the activation energy, separating the reactants and the products. – activation energy: amount of energy needed to disrupt stable molecule so that reaction can take place. Enzymes
Lower a
Reaction’s
Activation
Energy

What is the difference between an enzyme and a protein?

Protein Enzymes RNA

•All enzymes are proteins except some
RNAs

2) The active site of the enzyme
• Enzymes bind substrates to their active site and stabilize the transition state of the reaction.
• The active site of the enzyme is the place where the substrate binds and at which catalysis occurs.
• The active site binds the substrate, forming an enzymesubstrate(ES) complex.

Binding site
Active site
Catalytic site

Enzymatic reaction steps

1.
2.
3.
4.
5.

Substrate approaches active site
Enzyme-substrate complex forms
Substrate transformed into products
Products released
Enzyme recycled

6. Enzyme activity
• Enzymes are never expressed in terms of their concentration (as mg or μg etc.), but are expressed only as activities.
• Enzyme activity = moles of substrate converted to product per unit time.
– The rate of appearance of product or the rate of disappearance of substrate
– Test the absorbance: spectrophotometer

7. Factors affecting enzyme activity
•
•
•
•
•
•

Concentration of substrate
Concentration of enzyme
Temperature
pH
Activators
Inhibitors

Enzyme velocity
• Enzyme activity is commonly expressed by the intial rate (V0) of the reaction being catalyzed.
(why?)

• Enzyme activity = moles of substrate converted to product per unit time.

MichaelisMenten equation

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