Rki1p is a D-ribose-5-phosphate ketol-isomerase (EC:5.3.1.6) in the non-oxidative branch of the pentose phosphate pathway (1). In addition to the pentose phosphate pathway, Rki1p is also involved in pyridoxine biosynthesis. Pyridoxine is the major form of vitamin B6 and is a precursor for pyridoxal phosphate, an important coenzyme for amino acid metabolism (2). RKI1 is transcriptionally repressed in response to lithium chloride, and may be transcriptionally regulated in response to stress, as suggested by promoter analysis (3, 1). Active Rki1p is a tetramer and Rki1p is very unstable in crude extracts (2, 1). Rki1p localizes to the nucleus and cytoplasm (4).

rki1 null mutants are inviable, which suggests that Rki1p has an additional essential role other than its role in the non-oxidative part of the pentose-phosphate pathway since defects in genes encoding other enzymes of this part of the pentose phosphate pathway (rpe1 null mutants) are not lethal (2, 1). rki1-[R189K] mutants display a pyridoxine auxotrophy, and Rki1p-[R189K] mutant proteins contain 0.6% of the activity of Rki1p, appear to have an altered 3-dimensional structure, and form dimers instead of tetramers (2).

Rki1p has similarity to the ribose 5-phosphate ketol-isomerases from many prokaryotes and eukaryotes that include: Schizosaccharomyces pombe, Arabidopsis thaliana, Caenorhabditis elegans (B0280.3), Drosophila melanogaster, mouse (RPI) and humans. Mutations in the human homolog RPIA (OMIM) are associated with ribose 5-phosphate isomerase deficiency (OMIM; 2, 5, 1).

Last updated: 2005-12-14