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

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 three-dimensional 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 conditions- 37℃, physiological pH, ambient atmospheric pressure

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)

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

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

(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)

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

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

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

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

(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).

(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

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