TBL 2: Enzymes - Catalysis & Kinetics

Oxidoreductases

electron transfer (Hydrogens)

Transferases

transfer group other than H

(phosphate & amino & carbon)

Hydrolases

hydrolysis rxn cleaves bond after adding water

Lyases (synthase)

cleave C-C, C-O, C-N, & other bonds WITHOUT adding water

• often forms double bond

Isomerases

transfer of groups within a molecule (intramolecular transfer)

Ligases (synthetases)

bond formation coupled to ATP hydrolysis

kinase

catalyzes transfer of phosphate group from a high-energy molecule (ATP) to a substrate

Phosphorylase

adds inorganic phosphate onto a substrate without using ATP

*if it uses ATP, it’s a phosphatase

Dehydrogenase

catalyzes oxidation-reduction (redox) rxns

What are TS analogs?

Name some drug examples:

enzyme inhibitors that tightly bind to transition state (TS)

(bind better than natural substrate)

Ex1) Oseltamivir (Tamiflu)

Ex2) Abzymes [catalytic antibodies] against cocaine esterase

—> cocaine degradation

Cofactors

required by some enzymes for catalytic activity

• metal ions, organic or inorganic (zinc, iron, copper)

Coenzymes

organic cofactors, commonly derived from vitamins

Ex. flavin, heme

Cosubstrate

coenzymes that only transiently associate w/ the enzyme

Prosthetic group

tightly bound coenzyme (covalent/permanent bond)

ex. heme, biotin, FAD flavin, retinal

Metalloenzymes

inorganic cofactors that noncovalently associate w/ enzymes

• may help orient substrates in right direc

• or function as electron carrier

Ex. Fe2+, Mg2+

Apoenzyme

inactive protein portion (w/o coenzyme or cofactor)

Holoenzyme

whole, active enzyme (w/ coenzyme)

Proenzymes/Zymogens

inactive precursor form of an enzyme

• cleavage of specific peptide within proezyme generates active mature enzyme

Ex. Pepsinogen —> pepsin

Isozymes

enzymes that catalyze the same chemical rxn but differ in AA sequence

Ex. Hexokinase & Glucokinase

Michaelis-Menten model inapplicable when…

enzymes present in higher concentration than their substrates

Ethanol has ____ affinity for alcohol dehydrogenase [ADH] than methanol

GREATER [20x more]

• used to treat methanol & ethylene glycol toxicity (from alcoholism)

Competitive Inhibition

• Km increased

• Vmax unaffected

• binds to active site of free enzyme

Ex. Methotrexate [inhibits dihydrofolate reductase]

Noncompetitive Inhibition

• Km unaffected

• Vmax decreased

• binds to allosteric site of enzyme or ES complex

Ex. Acetazolamide

Uncompetitive Inhibition

• Km decreased

• Vmax decreased

• binds to allosteric site of ES complex

Irreversible Inhibition

• Km unaffected

• Vmax decreased

*mirrors noncompetitve inhibitors

• covalent modification

  • can only be overcome by synth. of new enzyme

Ex. Lead, organophosphates [malathion], cyanide, aspirin, penicillin, disulfiram

Lead

irreversible enzyme inhibitor

target enzyme:

• δ-aminolaevulinic acid (ALA0 dehydratase & ferro chelatase

*involved in synth of heme

clinical presentation/use:

• abdom pain. sideroblastic anemia, irritability, headache, impaired nervous system dvlpment

treatment:

• Ca-EDTA w/ dicamercaprol

Disulfiram

irreversible enzyme inhibitor

target enzyme:

• aldehyde dehydrogenase

clinical presentation/use:

• accumulation of acetaldehyde —> alcohol avoidance (treats alcoholism)

Penicillin

irreversible enzyme inhibitor; TS analog

target enzyme:

• transpeptidase

clinical presentation/use:

• antibiotic

Omeprazole & Lansoprazole

irreversible enzyme inhibitor

target:

• K+/H+ ATPase

clinical presentation/use:

• treatment of gastric ulcers

5-flourouracil

irreversible enzyme inhibitor

target enzyme:

• thymidylate synthasee

clinical presentation/use:

• anticancer agent

Allosteric regulation

• show cooperativity

  • sigmoidal cure

  • DOES NOT follow M-M kinetics

• enzymes w/cooperativity ALWAYS have multiple subunits

• allosteric effectors increase or decrease Km

Covalent Modifications

enzyme activity affected by addition or removal of phosphate groups from ser, thr, or tyr residues of enzyme

• phosphorylation one of primary ways

  • catalyzed by protein kinases

• phosphorylation can both activate or inactive enzymes

Acetozolamide

• noncompetitive inhibitor

• diuretic that inhibits carbonic anhydrase used in glaucoma & altitude sickness

Methotrexate

• competitive inhibitor

• inhibits dihydrofolate reductase