Chromogenic Substrates


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What is a chromogenic Substrate?

Proteolytic Enzymes

Serine Proteases

Proteolytic Mechanism

Specificity and Selectivity

Enzyme Kinetics

Substrates in Practice

Selectivity Tables

Kinetic Tables

Protein Concentrations in Plasma

Theoretical Basis for Calculation

International Units and Enzyme Activity




The proteolytic mechanism of serine proteases

The bond-cleaving reaction exerted by a serine protease on its substrate is the result of an interaction between the substrate and the charge relay network of the enzyme.
This network, which is present in the active site of all serine proteases, is known as the catalytic triad. 
It is built up from the side-chains of three specific amino acids (the hydroxy group of serine, the imidazole group of histidine and the carboxylic acid group of aspartic acid) that interact with each other through an array of hydrogen bonds.

Figure 1. Charge realy network of serine proteases.

The proteolytic action of a serine protease on its substrate comprises several steps starting with the formation of a non-covalent complex between the enzyme and the substrate. A nucleophilic attack by the serine hydroxyl group on the amide carbonyl carbon atom in the substrate results in cleavage of the amide bond and the formation of an acyl-enzyme intermediate.

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Figure 2. Formation of an acyl-enzyme intermediate.

The acyl-enzyme ester bond is then hydrolysed in the rate limiting step and the enzyme is now free to catalyze the cleavage of another substrate molecule.

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Figure 3. Hydrolysis of the acyl-enzyme intermediate.


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