2. Apoenzyme – the polypeptide portion of an enzyme
Cofactor – non protein portion of an enzyme
May be a metal ion such as Zn2+ of Mg2+
May also be an organic molecule such as vitamin B or heme – called a coenzyme
Substrate – the molecule an enzyme acts on
Activation – any process that initiates or increases the action of an enzyme
Inhibition – any process that inactivates an enzyme or reduces its effectiveness
Competitive inhibitor – binds at the active site
Non-competitive inhibitor – binds at other than the active site

3. Enzyme Catalysis Enzyme: a biological catalyst
with the exception of some RNAs that catalyze their own splicing, all enzymes are proteins
enzymes can increase the rate of a reaction by a factor of up to 1020 over an uncatalyzed reaction
some catalyze the reaction of only one compound
others are stereospecific; for example, enzymes that catalyze the reactions of only L-amino acids
others catalyze reactions of specific types of compounds or bonds; for example, trypsin that catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg

4. Mechanism of Action
both the lock-and-key model and the induced-fit model emphasize the shape of the active site
however, the chemistry of the active site is the most important
just five amino acids participate in the active sites in more than 65% of the enzymes studies to date
these five are His > Cys > Asp > Arg > Glu
four these amino acids have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH)

6. Enzyme Regulation
Protein modification: the process of affecting enzyme activity by covalently modifying it -
the best known examples of protein modification involve phosphorylation/dephosphorylation
example: pyruvate kinase (PK) is the active form of the enzyme; it is inactivated by phosphorylation to pyruvate kinase phosphate (PKP)

7. Succinylcholine is a competitive inhibitor of acetylcholine at receptors – it is broken down slowly by acetylcholinesterase. This leaves the receptor blocked and the muscle relaxed.
Chem Connect 22A, p.555

8. Sulfa drugs are competitive inhibitors of p-aminobenzoic acid in the synthesis of folic acid in bacteria
Chem Connect 22D, p.562

9. Enzyme Regulation
Proenzyme (zymogen): an inactive form of an enzyme that must have part of its polypeptide chain cleaved before it becomes active.
An example is trypsin, a digestive enzyme - it is synthesized and stored as trypsinogen, which has no enzyme activity. It becomes active only after a six-amino acid fragment is hydrolyzed from the N-terminal end of its chain Removal of this small fragment changes in not only the primary structure but also the tertiary structure, allowing the molecule to achieve its active form

10. Enzyme Regulation
Isoenzyme: an enzyme that occurs in multiple forms; each catalyzes the same reaction
Example: lactate dehydrogenase (LDH) catalyzes the oxidation of lactate to pyruvate The enzyme is a tetramer of H and M chains. H4 is present predominately in heart muscle. M4 is present predominantly in the liver and in skeletal muscle. H3M, H2M2, and HM3 also exist
H4 is allosterically inhibited by high levels of pyruvate while M4 is not. H4 in serum correlates with the severity of heart attack