Phosphoglyceric acid formation of carbon dioxide fixation in plant extracts

Abstract

Further work in the cell-free system from plant materials which forms phosphoglyceric acid by CO2 fixation has shown that the precipitate which was termed "enzyme" in a previous paper (Fager, 1952b) is in fact the source of the CO2 acceptor, and that the carboxylation enzyme is present in the "chloroplasts". The enzyme is dependent upon free sulphhydryl groups for activity and the carboxylation is inhibited by cyanide in a manner parallel to the effect of this poison on intact algae. A hot-water extract of pre- illuminated algae has been found to be a reliable source of the acceptor. When this extract is used, over 90% of the total fixation is found in the form of phosphoglyceric acid. The acceptor is a relatively stable phosphate ester. Vinyl phosphate, phosphoglycollic acid, phosphogylcolaldehyde and ketose phosphates do not function as acceptors in the system as studied. Evidence is presented to show that the fixation is not an exchange reaction involving the carboxyl group of an "active" phosphoglyceric acid. It is suggested that there is only a single photochemical reduction step in photosynthesis and that this step is the reduction of phosphoglyceric acid to the level of carbohydrate.