Active-site carbamate formation and reaction-intermediate-analog binding by ribulosebisphosphate carboxylase/oxygenase in the absence of its small subunits

Abstract

Even though depleted of more than 90% of its small subunits, ribulose 1,5-bisphosphate carboxylase/oxygenase from Synechococcus ACMM 323 still formed a stable complex with 2-carboxyarabinitol 1,5-bisphosphate from which exchange of the activator CO₂ molecule was prevented. The stoichiometry between nonexchangeable CO₂ and large subunits was unchanged regardless of the presence or absence of small subunits. The small-subunit-depleted enzyme was also “activated” by exposure to CO₂ and Mg²⁺, although it was necessary for the small subunits to be bound before this “activation” could be expressed. Binding of small subunits occurred rapidly, its rate depending on subunit concentration. The initial rate of “activation” was not slowed in the absence of small subunits but its extent at equilibrium was reduced. These observations are not consistent with an obligate role for the small subunits in the activation process. Their necessity in catalysis must stem from a more subtle involvement in the catalytic mechanism itself.