9. enzyme regulation

enzyme regulation mechanisms: Compartmentalization

Compartmentalization within specific organelles

(eg. enzymes involved in fatty acid synthesis located in the cytosol, those responsible for FA oxidation in mitochondria)

Changes in protein expression (by activation/inhibition of gene expression)

control of proteolysis

(eg. digestive enzymes synthesized as pro-enzymes (zymogens) and are activated by removal of inhibitory fragment)

Covalent modifications

Allosteric regulation

Control of degradation

covalent modifications

most common = phosphorylation

phosphorylation

catalyzed by kinases, use ATP as phosphate donor

Depending on the enzyme, phosphorylation can increase or decrease its activity
generally under hormonal control

Glycogen phosphorylase

activated by phosphorylation

Glycogen synthase

inactivated by phosphorylation

dephosphorylation

catalyzed by phosphatases

Allosteric regulation

Regulation by allosteric factors

A. Change affinity of enzyme for substrate (Km)
B. Modify catalytic activity (Vmax)

allosteric factors

molecules that bind non-covalently at a site different from
the active site and induce conformational changes in the active site

Allosteric modulators: homotropic effect

the substrate acts as allosteric regulator

Typically positive effectors

act by cooperativity (binding at one site, increases binding at other sites)

Allosteric modulators: heterotropic effect

the effector is different from the substrate

Can be positive or negative effectors

Generally act by feedback, where downstream products regulate upstream reactions

Allosteric enzymes

1. Catalyze mostly rate limiting reactions
2. Generally have multiple subunits
3. Do not follow Michaelis-Menten kinetics
4. Are regulated by allosteric activators or inhibitors
5. Activators and inhibitors do not need to have structural
similarity to the substrate structure
6. One enzyme can have one or multiple allosteric sites
7. The reaction is not affected by substrate concentration

Clinically relevant enzymes: Plasma enzymes

Actively secreted in blood (precursors of coagulation enzymes)

Released during cell turnover

Low and constant in healthy individuals

high concentrations of plasma enzymes can indicate

Tissue damage (concentration indicative of damage extent)

Liver damage: increased alanine aminotransferase (ALT)’

Increased cell proliferation

Isoenzymes

alternative enzyme forms (various subunit combinations) that can be separated electrophoretically and can catalyze similar reactions

Might be localized in different cellular compartments or different tissues

Lactate dehydrogenase **KNOW