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Biological molecules (proteins) that act as catalysts and help complex reactions.
are specific to their substrates and each enzyme has its own optimum pH
Material upon which an enzyme acts.
Enzyme inhibition
important in normal metabolism for control of pathways.
Reversible inhibitor
compet­itive inhibition
(Raises Km only) Same size and shape with the substrate
noncom­pet­itive inhibition
(Lowers Vmax only) inhibitor doesn't mind whether there is a substrate or not. but when the inhibitor binds, it switches off catalysis.
uncomp­etitive inhibition
(Lowers Vmax and Km) the inhibitor can ONLY be on the surface of the enzyme if the substrate is there.
Irreve­rsible inhibitor
acts by reacting with the enzyme protein, usually at the active site(s­ubs­trate site), to perman­ently block activity.
Vmax is the maximum rate of an enzyme catalysed reaction i.e. when the enzyme is saturated by the substrate.
Km is measure of how easily the enzyme can be saturated by the substrate.
Km and Vmax are constant for a given temper­ature and pH and are used to charac­terise enzymes. They can be used to identify types of inhibitors i.e. compet­itive, non-co­mpe­titive and uncomp­eti­tive.

Allosteric site


Normal binding

Enzyme kinetics

Reversible inhibition

Enzyme in human digestion

free energy, G


with co-enzyme


In the case of metal ion cofactors
Stabilise the structure, not directly involved in the chemistry
Ca++ with some protei­nases
part of substrate
Mg++-ATP with some kinases
part of active site
Zn++ in alcohol dehydr­ogenase
Organic cofactors
they do come on and off like other substrates (NAD+)
prosthetic groups
a cofactor that forms a permanent part of the enzyme's active site.
Doesn't come on and off in a catalytic cycle (FAD,PLP)

NAD+ is both a coenzyme and a substrate

Why use coenzyme in one case and not in the other case?
Metabolic point: NAD is one of a range of cofactor substances that is present in small concen­tration
They turn over and over again to process a large amount of substances
Pathway is to process large amount with tiny amount of coenzyme


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