An effector is a molecule (originally referring to small molecule

Small molecule

In the fields of pharmacology and biochemistry, a small molecule is a low molecular weight organic compound which is by definition not a polymer...

s but now encompassing any regulatory molecule, including proteins) that binds to a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 and thereby alters the activity of that protein. A modulator molecule binds to a regulatory site

Regulatory site

A regulatory site is a site on an allosteric protein to which a modulator molecule binds. A ligand-binding site on a receptor or enzyme distinct from the active site. Allosteric modulators alter enzyme activity by binding to the regulatory site. Also known as an "allosteric site"....

 during allosteric modulation and allosterically modulates the shape of the protein.

An effector can also be a protein that is secreted from a pathogen

Pathogen

A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

, which alters the host organism to enable infection, e.g. by suppressing the host's immune system capabilities.

Types of effectors

• Activators
• Inhibitor
  Enzyme inhibitor
  An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...
  
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