Regulation

Works at the protein level.
When the product of a metabolic pathway inhibits the activity of an enzyme involved in its synthesis (Fig 2.28)

Allosteric Regulation

allo = other
steric= site

The regulatory molecules bind not to the catalytic site (active site) but to a different site on the enzyme. Feedback inhibition is one example of this form of regulation.

Allosteric enzymes are usually made up of multiple subunits held together by quaternary  interactions.

Binding of the regulatory molecule changes the conformation of the protein, which in turn alters the shape of the active site and the catalytic activity of the enzyme. The regulatory molecule may serve as an activator of enzyme activity rather than an inhibitor.

For example, oxygen is carriedby myoglobin (a monomer) and hemoglobin (a tetramer of two alpha and two beta subunits).  The hemoglobin is an allosteric enzyme and can release more oxygen to venous tissue than myoblobin. (Fig. 1).

 Phosphorylation

Enzymes can also be regulated by covalent modifications such as Phosphorylation. Phosphorylation may stimulate or inhibit enzyme activity, depending on the individual enzyme.

 The phosphate for phosphorylation is supplied by ATP.