Measurement of k(on) without a rapid-mixing device.

We have recently designed a biochemistry laboratory experiment for the purpose of providing students an advanced experience with enzyme kinetics and the kinetics of binding. Bestatin, a well-known and commercially available general protease inhibitor, is a slow-binding inhibitor of aminopeptidase isolated from Aeromonas proteolytica. The binding is on a timescale slow enough for measurement without the use of a rapid-mixing device. Aminopeptidase inhibition is detected via a standard colorimetric assay with an inexpensive commercially available substrate. The binding of bestatin follows first order binding kinetics with a rate constant k(on) of 59 ± 5 M(-1) s(-1) . This aminopeptidase is well characterized with several crystal structures and a published K(i) , which students can then use to calculate the value for k(off) .