SN7 SHORT NOTE 5M

Describe the types of enzyme specificity with examples.

Biochemistry · 1st Professional — Pre-Clinical

Enzyme specificity = the ability of an enzyme to select and act on one substrate, or one type of bond, in preference to all others.

It is the property that distinguishes enzymes from chemical catalysts, and arises from the three-dimensional structure of the active site

Type	Meaning	Example
Absolute (substrate)	Acts on one substrate only	Urease → urea only; lactase → lactose only
Group specificity	Acts on a group of related substrates sharing a chemical group	Trypsin — peptide bonds on the carboxyl side of lysine and arginine; chymotrypsin — aromatic amino acids
Bond specificity	Acts on a type of bond, whatever the surrounding structure	Lipase → ester bonds; α-amylase → α-1,4 glycosidic bonds
Stereospecificity	Acts on one optical or geometric isomer only	L-amino acid oxidase; hexokinase acts on D-glucose but not L-glucose; fumarase acts on the trans isomer only

The active site is a three-dimensional cleft whose shape, charge distribution and hydrophobicity complement the substrate
Lock and key (Fischer) — rigid complementarity
Induced fit (Koshland) — the site moulds itself around the substrate; the accepted model
Three-point attachment explains stereospecificity — only one enantiomer can contact all three binding points simultaneously

Prevents unwanted side reactions in a cell containing thousands of compounds
Allows precise regulation of individual pathways
Makes enzymes valuable as laboratory reagents — glucose oxidase measures glucose without interference from other sugars
Provides the basis for selective drug action

Glucose oxidase strips for blood glucose depend on absolute specificity; older copper-reduction methods (Benedict) detect any reducing sugar and are therefore positive in galactosaemia and pentosuria too
Stereospecificity in pharmacology — drug enantiomers may differ entirely in effect; the tragedy of thalidomide is the classical example
Enzyme replacement therapy works only because the administered enzyme is specific for the accumulated substrate, as in Gaucher disease
Loss of specificity does not occur — but a mutation altering the active site abolishes activity, which is how most inborn errors arise
Broad-specificity enzymes have their uses — cytochrome P450 acts on a huge range of drugs, which is why so many drug interactions occur at this site

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