The utilisation of gluconate by Escherichia coli K12.
thesisposted on 19.11.2015, 09:08 by Pelin. Faik
Many micro-organisms catablise gluconate via the Entner-Doudoroff pathway. The key enzymes of this pathway are gluconate kinase, 6-phosphogluconate dehydratase (Edd) and 2-keto 5-deoxy 6-phosphogluconate (KDPG) aldolase. Biochemical and genetic techniques have been used to study gluconate utilisation in Escherichia coli K12. Mutants of the pathway have been isolated, their genetic lesions mapped and their physiological effects studied. The first step in gluconate utilisation by E. coli is its entry into the cells. It has been established that uptake is an active process induced by growth on gluconate, and appears to be the rate-limiting step in gluconate utilisation. Gluconate thus taken up is then phosphorylated to 6-phosphogluconato by ATP, catalysed by gluconate kinase: mutants completely devoid of this enzyme have not been obtained and it may be that there are two gluconate kinases in E. coli. 6-phosphogluconate can only be metabolised via the Entner-Doudoroff and pentose-phosphate pathways since Edd- mutants that also lack 6-phosphogluconate dehydrogenase (Gnd-) do Dot grow on gluconate but the Entner-Doudoroff pathway plays the predominant role. The importance of the KDPG aldolase that catalyses the cleavage of KDPG to pyruvate and glyceraldehyde 3-phosphate has been studied with mutants devoid of this enzyme. It has been established that KDPG is a very effective competitive inhibitor of 6-phosphogluconate for Gnd. The enzymes involved in the catabolism of gluconate are inducible. The likely inducer for the uptake system and gluconate kinase(s) is gluconate itself; 6-phosphogluconate probably induces Edd and Kga. Genetic analysis shows that at least three regions of coli chromosome (at 36, 66 and 85 min) contain genes involved in gluconate utilisation; only some of these genes are linked.