Glyceraldehyde-3-phosphate Dehydrogenase and Fructose-1, 6-bisphosphatase of the Enteric Pathogens C. jejuni and H. pylori

Campylobacter jejuni and Helicobacter pylori are pathogens which cause gastrointestinal diseases and are therefore of significant importance. However, their metabolism and physiology is relatively poorly understood. It had been noted that the genome of these pathogens lack open reading frames for some glycolytic enzymes. Notably, both pathogens lack the gene encoding phosphofructokinase (6-PFK) and thus regulation of the complementary gluconeogenic enzyme fructose-1,6-bisphosphatase (FBPase) might be different to that in the majority of organisms which retain 6-PFK. In order to further understand the metabolism of C. jejuni and H. pylori, the structure and function of FBPase and the key gluconeogenic/glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was investigated. Specifically, the regulation of FBPase in both pathogens was investigated using kinetic and biophysical techniques. The results suggested that the enzymes are insensitive to AMP inhibition, unlike mammalian and E. coli FBPases. The FBPases were also insensitive to the other compounds of glycolytic and gluconeogenic pathways. The essentiality of fbp in C. jejuni was also tested and confirmed with gene complementation methodology. The essentiality and difference in regulation of these enzymes suggests they have potential as drug targets. The crystal structures of C. jejuni GAPDH (cjGAPDH) with bound NAD+ and NADP+ showed dual coenzyme specificity, revealing similarities with plant GAPDHs and suggesting a gluconeogenic role. The mechanism of inhibition of cjGAPDH was investigated further through covalent modification of the active site cysteine by iodoacetamide, this blocked NAD(P)+ binding. A competitive inhibitor-bound structure of cjGAPDH, in which the coenzyme was replaced by ADP, was also solved. Comparison of the crystal structures of cjGAPDH-ADP and cjGAPDH-NAD(P)+ complexes highlighted specific conformational changes linked to interactions with the ribose 2’-phosphate. The interactions of this 2’-phosphate might also be utilised to inform the design of lead compounds for inhibitory drug development.