Pea carbonic anhydrase : a kinetic study

Abstract: The enzyme carbonic anhydrase (CA), catalysing the interconversion between CO2 and HCO3', has long been known to be present in plants as well as in animals. Several of the animal isozymes, but none of the plant CAs, have been extensively studied. When the first plant CA cDNA sequences were published in 1990, it was obvious that the animal and plant CAs represent evolutionarily distinct families with no significant sequence homology between the families.Pea CA is synthesised as a precursor and subsequently processed at the import into the chloroplast. When we purified CA from pea leaves two oligomeric forms with molecular masses around 230 kDa were obtained. One form was homogenous while the other form contained subunits of two different sizes. The larger subunit has an acidic and highly charged N-terminal extension, consisting of 37 residues. We propose that the sequence that precedes the cleavage site resulting in the large subunit represents the functional transit peptide, directing CA to the chloroplast. Neither the transit peptide nor the acidic 37-residue peptide were found to affect the folding, activity or oligomerisation of pea CA.Kinetic investigations showed that pea CA requires a reduced environment and high concentrations of buffer for maximal catalytic activity. High buffer concentrations result in a faster turnover of the enzyme (kcat) while the efficiency (kcatlKm) is not affected. This is consistent with a ping-pong mechanism with the buffer as the second substrate. Both kcat and kcatlKm increase with pH but the dependences cannot be described by simple titration curves. SCN' is an uncompetitive inhibitor at high pH and a noncompetitive inhibitor at neutral and low pH. This is in accordance with the mechanistic model, previously proposed for human CAM, involving a zincbound water molecule as a catalytic group. In this model, the carbon dioxide - bicarbonate interconversion, reflected by kcatlKm, is temporally separated from a rate limiting proton-transfer step. At high pH, solvent hydrogen isotope effects obtained for pea CA agree with this scheme, while they do not fit at neutral and low pH.Site-specific mutations of cysteine residues at positions 165, 269 and 272 were difficult to study, either because strong deviations from Michaelis-Menten kinetics were observed, or because the mutants were found in inclusion bodies. However, the mutant H208A was found to be a very efficient enzyme with the highest kcatlKm value obtained for any CA so far, 2.9-108 M'1s '1. With the H208A mutant an increased dependence on high buffer concentrations at low pH was obtained. At high pH, the mutant is more efficient than the unmutated enzyme. The H208A mutant is also more prone to oxidation than the wild-type enzyme.