Chapter 6

Substrate-level regulation

A means of regulation related to access to the substrate.

prosthetic group

A nonprotein group forming part of or combined with a protein.

Allosteric (or regulatory) site

A site that binds another molecule that affects the enzyme's regulation. When a molecule binds an allosteric site, it alters the enzyme's shape, or conformation, which then changes how the enzyme functions.

substrate specificity

Ability of an enzyme to discriminate between very similar molecules. The property that explains why enzymes have very few substrates and many only one substrate

protein kinase

An enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein.

regulatory subunit

The polypeptide in an enzyme protein that does not contain the active site, but instead binds non-substrate molecules and changes its structure, in turn changing the structure and function of the active site. Contains the allosteric site. (Contrast catalytic subunit.)

Enzymes

Proteins that act as biological catalysts.

catalytic subunit

Subunit containing the active site of an enzyme.

allosteric regulation

The binding of a regulatory molecule to a protein at one site that affects the function of the protein at a different site.

Phosphorylation

An important mechanism by which the activity of proteins can be altered after they are formed. A phosphate group ( PO3−4 ) is added to a protein by specific enzymes called kinases. This phosphate group is usually provided by ATP, the energy carrier of the cell.

Ribozymes

Catalytic RNA molecules that function as enzymes.

Sensitivity to pH

Changes in pH may not only affect the shape of an enzyme but it may also change the shape or charge properties of the substrate so that either the substrate cannot bind to the active site or it cannot undergo catalysis. Enzymes have an optimum pH.

induced fit model

Enzyme model where the substrate induces the enzyme to alter its shape slightly so it fits better

allosteric enzymes

Enzymes that experience changes in their conformation as a result of interactions at sites other than the active site called allosteric sites; conformational changes may increase or decrease enzyme activity

Phosphatases

Enzymes that remove phosphate groups from proteins.

covalent modification

When proteins have different groups covalently attached to them to regulate their activity. Ex. phosphorylation to hydroxyl group of serine, threonine, or tyrosine by kinase or phosphorylase

Allosteric inhibitors

bind to the allosteric site to inactivate the enzyme

allosteric activator

binds to allosteric site and increases enzyme activity