Enzyme regulation

What are enzyme inhibitors

Molecules that decrease enzyme activity

Some are natural and some are synthetic

Reversible inhibition

Inhibitor binds non-permanently to the enzyme

Enzyme function can be restored

Often resembles enzyme’s substrate

Can bind to free enzyme (prevent substrate binding) or to ES complex (prevent reaction completion)

Irreversible inhibition

Inhibitor permanently alters the enzyme

Forms covalent bonds

Making it nonfunctional

Competitive inhibitors

Compete with substrate for active site

Structurally similar to substrate

Reduces substrate binding

Does not affect catalysis

Uncompetitive inhibitors

Bind at separate site than active site

Binds after substrate has attached to enzyme

Prevents enzyme from catalysing reaction

Mixed inhibitors

Binds to both free enzyme and ES complex

Inhibits both substrate binding and catalysis

Competitive inhibition plot

Vmax (maximum reaction velocity) - remains unchanged (substrate can outcompete the inhibitor and saturate the enzyme)

Km - increases (enzyme affinity for substrate decreases), higher Km means lower affinity

Uncompetitive inhibition plot

Km - decreases (higher affinity as enzyme holds substrate better as it is locked in place)

Vmax - decreases (inhibitor prevents reaction, fewer enzymes available to convert substrate to product, therefore can’t reach full speed)

Mixed inhibition plot

Vmax - dcreases (always reduced the number of function enzymes, slows down reaction)

Km - unpredictable (if bind to free enzyme → Km increases like competitive inhibition, or bind to ES complex → Km decreases like uncompetitive)

Allosteric modulation

Enzyme activity is controlled by molecules that bind to a site other than active site

Positive (increase activity) or negative (decrease activity)

Homotropic (modulator is substrate) or heterotropic (modulator diff molecule)

Covalent modification

Enzyme regulated through addition/removal of chemical groups

Phosphorylation

Can activate or inactivate enzymes

Phosphate group is added to protein or enzyme

Enzymes involved - kinase (add phosphate) and phosphatase (remove phosph)

Basic reaction -

Protein + ATP → Phosphorylated protein + ADP

• Phosphate group comes out of ATP

Zymogen

Inactive form of an enzyme

Requires cleavage of spcific speptide bonds to become active

Helps prevents premature enzyme activity, rapid activation and regulation

Zymogen activation

Proteolytic cleavage - small part of protein is cut off, causing conformational change that activates enzyme

Some examples include digestive enzymes

Phosphorylation ACTIVATES enzyme

Phosphorylated enzyme has higher activity

• Curve is shifted upwards (higher Vmax)

• Enzyme reaches Vmax faster at lower substrate concentration

• Km might decrease → higher affinity

Phosphorylation INHIBITS enzyme

Phosphorylated enzyme shows lower activity

• Curve is lower (lower Vmax)

• Needs more substrate to reach half maximal velocity (higher Km, lower affinity)