Figure 3.

The pH dependence of the proteasomal chymotrypsin-like (violet), trypsin-like (blue), and caspase-like (red) peptidase activities in the buffers containing HEPES (a), imidazole (b), spermidine (c), and spermine (d). Activity of the 26S proteasome was measured using Suc-LLVY-AMC (chymotrypsin-like), Ac-RLR-AMC (trypsin-like) or Ac-GPLD-AMC (caspase-like) fluorogenic substrates at 25 mM of the HEPES and imidazole buffer systems and 5 mM of the spermidine and spermine buffer systems. Relative activity was calculated as the ratio of activity at a distinct pH to the maximal activity within the tested pH range. The data represent the average and standard deviation (error bars) from four independent measurements. The data were fitted to a polynomial square of an exponential or Gauss function. The physiologically relevant pH range is shown by a bold line.