Enzyme Regulation and Allosteric Control

Allosteric enzymes can be studied using kinetics to see their effects.
Allosteric modulators are not necessarily binding at the enzyme's active site; some even bind substrates.
Substrate binding can change the enzyme's conformation, affecting subsequent substrate binding (positive or negative modulation).

Enzyme activity can be positively or negatively modulated.
Inhibitor design must account for involvement in multiple pathways.
Kinases transiently form covalent bonds during phosphorylation.
Enzymes may require processing to become active (e.g., chymotrypsin, insulin).
Activity regulation can be fine-tuned (pH changes) or irreversible (covalent bond formation).
Reactions requiring quick responses often involve covalent bond formation or cleavage.

Responses can be homotropic (substrate-related) or heterotropic (different compound).
Conformational changes occur between tense (T) and relaxed (R) states.
Allosteric modulators are distinct from uncompetitive and mixed inhibitors.
Allosteric modulators usually require a multimeric protein (dimer).

ATCase is involved in pyrimidine biosynthesis.
It transfers a carbamoyl group to aspartate.
Has 12 polypeptides forming 6 catalytic and 6 regulatory subunits.
Senses ATP (positive regulator) and CTP (negative regulator).
CTP inhibits by changing enzyme conformation when CTP levels are high.
ATP promotes the reaction; competition between ATP and CTP binding affects activity.
Kinetic curves change shape (sigmoidal to hyperbolic) without altering $V<em>{max}$ , only $K</em>m$ .

Examples: phosphorylation, adenylation, acetylation, myristoylation, ubiquitylation, ribosylation, methylation.
Each modification adds different properties (charge, hydrophobicity).
Phosphorylation by kinases is common but challenging to inhibit due to ATP binding.
Inhibitors can mimic the protein substrate to target specific kinases by recognizing the amino acid pattern around the phosphorylation site.

Involves cleavage of fibrinogen to form a fibrin mesh, stopping bleeding.
Irreversible cascade with multiple serine proteases activating downstream factors.
Thrombin plays a role in cleaving different factors for a quick response.