1. Dr.Sunita Adhikari (Nee Pramanik)
Enzyme
Dr.Sunita Adhikari (Nee Pramanik)

2. 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.”

3. 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 three-dimensional shapes that fit the shapes of reactants (substrates)

4. 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 conditions- 37℃, physiological pH, ambient atmospheric pressure

5. 3. Chemical composition of enzymes
(1) Simple protein
(2) Conjugated protein
Holoenzyme= Apoenzyme+ Cofactor
Coenzyme : loosely bound to enzyme (non-covalently bound).
Cofactor
Prosthetic group : very tightly or even covalently bound to enzyme (covalently bound)

6. 4. Classification of enzymes
(1). By their composition
1). Monomeric enzyme
2). Oligomeric enzyme
3). Multienzyme complex: such as Fatty acid synthase

7. Classification of Enzymes
Enzymes are classified according to the type of reaction they catalyze:
Class Reactions catalyzed
Oxidoreductases Oxidation-reduction
Transferases Transfer groups of atoms
Hydrolases Hydrolysis
Lyases Add atoms/remove atoms to/from a double bond
Isomerases Rearrange atoms
Ligases Use ATP to combine molecules

8. (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 classification)
By the reactions they catalyze (Six classes)

9. 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.

10. Enzymes Lower a Reaction’s Activation Energy
Energy and Enzymes
9/5/2018
Enzymes Lower a Reaction’s Activation Energy
G. Podgorski, Biol. 1010 11

11. What is the difference between an enzyme and a protein?
RNA
Enzymes
All enzymes are proteins except some RNAs
not all proteins are enzymes

12. 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 enzyme-substrate(ES) complex.
Binding site
Active site
Catalytic site

13. Enzymatic reaction steps
1. Substrate approaches active site
2. Enzyme-substrate complex forms
3. Substrate transformed into products
4. Products released
5. Enzyme recycled

14. 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

15. 7. Factors affecting enzyme activity
Concentration of substrate
Concentration of enzyme
Temperature pH
Activators
Inhibitors

16. (3) Effect of temperature on velocity
Bell-shaped curve

17. (4) Effect of pH value on velocity
Bell-shaped curve
Each enzyme has an optimal pH or pH range (where the enzyme has maximal activity).
Requirements for the catalytic groups in the active site in appropriate ionization state is a common reason for this phenomenon.
The pH-activity profiles of two enzymes. These curves are constructed from measurements of initial velocities when the reaction is carried out in buffers of different pH. Because pH is a logarithmic scale reflecting tenfold changes in [H+], the changes in V0 are also plotted on a logarithmic scale. The pH optimum for the activity of an enzyme is generally close to the pH of the environment in which the enzyme is normally found. Pepsin, which hydrolyzes certain peptide bonds of proteins during digestion in the stomach, has a pH optimum of about 1.6. The pH of gastric juice is between 1 and 2. Glucose 6-phosphatase of hepatocytes (liver cells), with a pH optimum of about 7.8, is responsible for releasing glucose into the blood. The normal pH of the cytosol of hepatocytes is about 7.2.
The pH optimum varies for different enzymes.
Most enzyme: neutral pH (6-8).

18. (5) Effect of activator on velocity
Enzyme activators are molecules that bind to enzymes and increase their activity.
(i). Inorganic ions
Metal ions，such as Na+, K+, Mg2+, Ca2+, Cu2+, Zn2+, Fe2+ et al
Anions: such as Cl-, Br-, I-、CN- et al
(ii). Organic
Reducing agents, such as Cys、GSH
(iii). Proteins

19. THANK YOU