Chapter 6-Enzymes (1) PDF

Enzymes Classes of Enzymes Nomenclature and Classification Enzymes are often classified by placing them in categories according to the reactions that they catalyze: 1. 2. 3. 4. 5. 6. Oxidoreductase Transferase Hydrolase Lyase Isomerase Ligase Oxidoreductases Catalyze oxidation-reduction reactions of...

Enzymes Classes of Enzymes Nomenclature and Classification Enzymes are often classified by placing them in categories according to the reactions that they catalyze: 1. 2. 3. 4. 5. 6. Oxidoreductase Transferase Hydrolase Lyase Isomerase Ligase Oxidoreductases Catalyze oxidation-reduction reactions of substrate molecules, most commonly addition or removal of O or H Require coenzymes that are reduced or oxidized as the substrate is oxidized or reduced Ex: alcohol dehydrogenase Transferases Catalyze the transfer of a groups from one molecule to another – Transaminases transfer an amino group between substrates – Kinases transfer a phosphate group from ATP to give ADP and a phophorylated product Hydrolases Catalyze the hydrolysis of substrates, i.e. the breaking of bonds with addition of water Important for digestion – Proteins are broken down to single amino acids – Carbohydrates are broken down to simple sugars Isomerases Catalyze the isomerization (rearrangement of atoms) of a substrate in reactions One substrate, one product Ex: triose phosphate isomerase Lyases Catalyze the addition of a molecules such as H2O, CO2, or NH3 to a double bond or the reverse reaction in which a molecule is eliminated to leave a double bond Ex: fumarase Ligases Catalyze the bonding together of two substrate molecules Not favorable, require simultaneous release of energy by hydrolysis reaction (i.e. conversion of ATP to ADP) Involved in the synthesis of proteins and DNA Classification of Enzymes 1. Oxidoreductase: O CH3 -C- COO Pyruvate - + NADH + H + lactate dehydrogenase OH + CH3 -CH-COO - + NAD Lactate 2. Transferase: COOCH2 CH- NH3 + + COO- COOAspartate amino C= O transferase CH2 CH2 COOCH2 C= O + COO- COOAspartate -Ketoglutarate COOC-N H3 + CH2 CH2 COO- Oxalosuccinate Glutamate 3. Hydrolase: O CH3 -C- OCH 2 CH 2 N( CH3 ) 2 + H2 O Acetylcholinesterease Acetylcholine O CH3 -C- O- HOCH2 CH2 N( CH3 ) 2 Acetate Choline Classification of Enzymes 4. Lyase: COOCH2 C-COO - + H O 2 CH COOcis- Aconitate COOCH2 H C-COO - Aconitase HO C-H COOIsocitrate 5. Isomerase: CH2 OPO 3 2 Phosphohexose O isomerase OH OH HO OH  - D- Glucose-6-phosphate 6. Ligase: CH2 OPO 3 2 O H HO H H HO CH2 OH OH  - D-Fructose-6-phosphate ATP + L-tyrosine + t-RNA Tyrosine-tRNA synthetase L-tyrosyl-tDNA + AMP + PP i Reaction Coordinate Diagram Comparing Enzyme-Catalyzed and Uncatalyzed Reactions Any reaction may have several steps, involving the formation and decay of transient chemical species called reaction intermediates. When several steps occur in a reaction, the overall rate is determined by the step (or steps) with the highest activation energy; this is called the rate-limiting step. GM = difference between transition state energies of uncatalyzed and catalyzed reactions Models for the binding of a substrate to an enzyme 1) The lock-and-key model Models for the binding of a substrate to an enzyme 2) The induced-fit model General mode of catalysis in enzymatic reactions: Effect of Substrate Concentration on Enzyme Activity Effect of Enzyme Concentration on Enzyme Activity Effect of Temperature on Enzyme Activity Effect of temperature and pH on enzyme activity Amino acid groups involved in catalytic mechanisms