The gluconate shunt is an alternative route for directing glucose into the pentose phosphate pathway in fission yeast

Abstract

Glycolysis and the pentose phosphate pathway both play a central role in the degradation of glucose in all domains of life. Another metabolic route that can facilitate glucose breakdown is the gluconate shunt. In this shunt glucose dehydrogenase and gluconate kinase catalyze the two-step conversion of glucose into the pentose phosphate pathway intermediate 6-phosphogluconate. Despite the presence of these enzymes in many organisms, their only established role is in the production of 6-phosphogluconate for the Entner-Doudoroff pathway. In this report we performed metabolic profiling on a strain of <i>Schizosaccharomyces pombe</i> lacking the zinc-responsive transcriptional repressor Loz1 with the goal of identifying metabolic pathways that were altered by cellular zinc status. This profiling revealed that <i>loz1</i>Δ cells accumulate higher levels of gluconate. We show that the altered gluconate levels in <i>loz1</i>Δ cells result from increased expression of <i>gcd1</i> By analyzing the activity of recombinant Gcd1 <i>in vitro</i> and by measuring gluconate levels in strains lacking enzymes of the gluconate shunt we demonstrate that Gcd1 encodes a novel NADP⁺-dependent glucose dehydrogenase that acts in a pathway with the Idn1 gluconate kinase. We also find that cells lacking <i>gcd1</i> and <i>zwf1</i>, which encode the first enzyme in the pentose phosphate pathway, have a more severe growth phenotype than cells lacking <i>zwf1</i> We propose that in <i>S. pombe</i> Gcd1 and Idn1 act together to shunt glucose into the pentose phosphate pathway, creating an alternative route for directing glucose into the pentose phosphate pathway that bypasses hexokinase and the rate-limiting enzyme glucose-6-phosphate dehydrogenase.