Effect of macromolecular crowding upon the structure and function of an enzyme: glyceraldehyde-3-phosphate dehydrogenase.

The specific activity of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (GAPD) has been measured as a function of GAPD concentration in the absence and presence of 18 g/dL ribonuclease A. The specific activity of GAPD at fixed concentration has been measured as a function of the concentration of added ribonuclease A, beta-lactoglobulin, bovine serum albumin, and poly(ethylene glycol) (Mr 20000) at additive concentrations of up to 30 g/dL. All of the data may be semiquantitatively accounted for by a simple model based upon the following qualitative assumptions: (1) Under the conditions of the reported experiments, GAPD exists primarily as an equilibrium mixture of monomers and tetramers of GAPD subunits. (2) The monomers have a much larger specific activity than do the tetramers. (3) The addition of high concentrations of unrelated globular proteins does not affect the activity of either monomer or tetramer but does promote the formation of tetramer due to space-filling properties of the added species, as proposed by Minton [Minton, A. P. (1981) Biopolymers (in press)].