Chapter 13: Enzyme
Enzyme: Biological catalyst (normally a protein) that functions to speed up the rate
of a biological reaction, but is not altered or consumed by the rxn.
 Have high catalytic power-catalyze fast
 Very specific-catalyze a single rxn
 Require activation energy
 Speed up rxn by lowering amt of activation required to start rxn.
 Bring rxn to equilibrium rather than completion
 Often have co-factors- non-protein components important in catalysis
o Organic (CoA, NAD, NADP, Prosthetic Groups[ Heme, Biotin])
o Inorganic (Zn**, Mg**)
Models for Enzyme Action:
1. Lock & Key: enzyme active site perfectly matches the shape of the substrate,
only allows 1 substrate to bind to the active site and be converted to product
2. Induced Fit: Substrates fit into active site, both enzyme and substrate
change shape. Enzymes are not as rigid as one thought, more flexible and
dynamic, has been confirmed by X-ray crystallography
Types of Kinetic Energy:
1. vibrational
2. rotational
3. translational
Substrates (S): the substances upon which an enzyme acts (reactants)
Products: the substances produced by chemical modification of substrates
Active Site: the specific region on/in an enzyme where substrates bind and where
the catalytic rxn occurs.
Transition State: the unstable (most energized) intermediate formed in an
enzymatic rxn that has properties of both the substrate and the product.
-stabilized by enzyme
Activation Energy: the threshold energy required to produce a chemical rxn.
Free Energy: difference in energy b/t products and reactants.
Six Classes of Enzymes:
1. Oxidoreductases: catalyze oxidation-reduction rxns; add/remove
electrons/protons from its substrate.
o Includes dehydogenases, oxidases, reductases, peroxidases.
2.
3.
4.
5.
6.
o Asians have worse hangovers b/c have more active form of Alcohol
dehydrogenase.
o Acetaldehyde is toxic, dehydrogenase produces acetyl-CoA
Transferases: transfer a functional group from one molecule to another
o Includes: amino, phosphoryl, methyl, acyl
o ex: hexokinase
Hydrolases: cleave bonds by adding a water molecule
o Includes: esterases, phosphatases, peptidases, lipases, glycosidases)
o ex: Phospholipase A2.
Lyases: remove functional groups via non-hydrolytic rxns. Often result in
double bond,
o Includes: decarboxylases, deanimases, dehyrateses
o Ex: phenylalanine Ammonia Lyase
Isomerases: catalyze rearrangements of functional groups within a molecule
o Mustases: transfer functional groups from one position to another
o Epimerases: invert functional groups about asymmetric carbons.
o Ex: Triose Phosphate Isomerase
Ligases: use the energy from ATP hydrolysis to form bonds b/t two
substrate molecules
o Form C-C, C-S, C-O & PO32- ester bonds
o Ex: Acetyl-CoA Carboxylase
Enzyme Commission Nomenclature (EC Numbers): Every enzyme has a unique #
series that precisely describes its chemical rxn.
EC 1: oxidoreductases
EC 2: transferases
EC 3: hydrolases
EC 4: lyases
EC 5: isomerases
EC 6: ligases