Tetra- and Nonapeptidyl Motifs in the Origin and Evolution of Photosynthetic Bioenergy Conversion
In this session on “Life without starlight” it may be useful to describe photosynthetic organisms with their light-induced transformation, conversion and conservation of energy as “life with starlight”. To what extent could photophosphorylation, this fundamental process of energy transfer in photosynthesis, have originated and evolved in discrete molecular steps in the absence of light? An answer may be sought in the first and so far only known alternative biological photophosphorylation system, where the comparatively “primitive” energy donor inorganic pyrophosphate (PPi) is produced (H. Baltscheffsky et al., 1966) instead of the more “complex” adenosine triphosphate (ATP), the well known central energy donor and “energy currency” of all now living cells. The gene for PPi synthase, which is the enzyme involved in the photosynthetic formation of PPi in chromatophores from the photosynthetic bacterium Rhodospirillum rubrum, has been cloned (M. Baltscheffsky et al., 1998) and the primary structure and some other properties of this first bacterial representative of the membrane-bound family of proton-pumping inorganic pyrophosphatases (H+-PPases) has been deduced (M. Baltscheffsky et al., 1999). In the loop between the fifth and sixth putative transmembrane segments, in what appears to be a pyrophosphate binding region and the active site for the phosphorylation reaction, three different “primitive” tetrapeptidyl motifs have been located in two nonapeptidyl sequences. Their high content of the four “very early” proteinaceous amino acids glycine, alanine, valine and aspartic acid (G, A, V and D, respectively) and their very regular arrangement of amino acids with charged side groups, would seem to be of particular evolutionary significance. Our first tentative model of the active site has resulted in a structure, which may be useful for attempts to extrapolate back to the hither toelusive molecular origin and evolution of the metabolism of phosphate compounds, specially those, which are directly involved in bioenergy conversion.
KeywordsPhosphate Compound Inorganic Pyrophosphatases Molecular Origin Inorganic Pyrophosphate Guanosine Diphosphate
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