Schnarrenberger, Claus

Plant cells contain two full sets of isoenzymes for the reactions of glycolysis, gluconeogenesis and for the oxidative pentose cycle. In higher plant cells, one set is located in plastids and the other set in the cytosol. Both sets of isoenzymes appear to be coded for by different gene loci on the nuclear DNA. Extensive structural differences between these isoenzymes are manifested in their kinetic properties, the small degree of immunochemical crossreactivity, large differences in the peptide pattern after tryptic digestion, and their size. To answer the question for the origin of the cytosol and plastid specific isoenzymes of the sugar phosphate metabolism, one may confer to an endosymbiontic model in which one set of isoenzymes originated from an eukaryotic precursor cell and the other set from a prokaryotic endosymbiont. A yet unknow transfer of genes in early evolution may account for the exclusive gene location on nuclear DNA of recent plants Immunochemical differences between homologous enzymes from algae and higher plants imply large structural changes of these isoenzymes during the evolution within the plant kingdom. Because of this,additional analyses with algal enzymes will be necessary to answer the question on the origin of the isoenzymes. At present an endosymbiontic origin of cytosol and plastid specific isoenzymes of the sugar phosphate metabolism is very likely, however, evidences are not fully conclusive yet.

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