Mobility of enzymes on insoluble substrates: The β‐amylase–starch gel system

The movement of enzymes along the surfaces of biopolymers containing enzyme‐susceptible sites can be described as a lateral diffusion process characterized by an apparent diffusion coefficient [E. Katchalski‐Katzir, J. Rishpon, E. Sahar, R. Lamed, and Y. I. Henis (1985) Biopolymers 24, 257–277]. Studies on the diffusion of enzymes on biopolymer substrates can therefore provide important information on the mechanism of enzyme–biopolymer interaction. For this reason, the motion of fluorescently labeled β‐amylase [α‐D‐(1 → 4)glucan maltohydrolase; E.C. 3.2.1.2] on the surface of starch gels was studied by fluorescence photobleaching recovery. The results indicate that the motion of β‐amylase on the surface of the gel substrate occurs by both lateral diffusion along the surface (over micron distances) and exchange between bound and free enzyme molecules in the solution covering the gel, and that the two processes occur concomitantly and in a random manner. Surface diffusion also appears an important process with respect to the action of the enzyme on the substrate sites, since this component of the motion disappears upon inactivation of the enzyme, leaving only exchange to contribute to the measured motion.