Kinetics of ribulosebisphosphate carboxylase at high protein concentration

Kinetic parameters of ribulos-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase) are usually evaluated in dilute solutions (less than 0.1 mg ml-1). Yet, this enzyme occurs in vivo at 100-200 mg ml-1 and a total protein concentration 300-400 mg ml-1. Enzymes can change their catalytic properties upon 'crowding'. Hence it became of interest to determine whether RuBP carboxylase elicits any properties not observable in dilute solution. Pre-steady state progress curves of fully activated enzyme showed an initial burst followed by a slower rate of product formation. The extent of the burst increased as concentration ratios of RuBP and RuBP carboxylase decreased. The burst corresponds to 1/8 turnover per holoenzyme or 1 turnover per active site. No discontinuity in progress curves was observed with partially activated enzyme.