![]() ![]() GmhA, a key enzyme along the heptose biosynthesis, has been identified as a novel target for antimicrobial intervention towards Gram-negative bacteria 18, 19.ĭevelopment of high affinity inhibitors against GmhA requires a firm understanding of its mechanism. Inhibitors of heptose biosynthesis could therefore potentiate other antibiotics. In particular, many Gram-negative bacteria incorporate heptoses into their lipopolysaccharide (LPS) core, and mutants with disrupted heptose production are highly susceptible to antibiotics 16. This product is then further modified to provide bacteria with heptoses for the production of polysaccharides. These tetrameric proteins catalyze the conversion of the pentose phosphate pathway intermediate sedoheptulose-7-phosphate (S7P) into d- manno-heptopyranose-7-phosphate (Supplementary Fig. One potentially interesting set of SIS enzymes are the heptose isomerases (GmhA) 14, 16, 17. This wide range of potential routes to control activity, and the differing numbers of subunits in different proteins (these enzymes are all either dimers or tetramers), offer evolution the opportunity to fine-tune the activity of SIS enzymes to cellular need. This SIS family of enzymes show an interesting range of active site chemistries: the same evolutionarily conserved scaffold is used to coordinate different active site side chains to drive a variety of reactions throughout the family. They generally bind to sugar phosphates, using the phosphate to provide affinity, and catalyzing reactions at other positions in the sugar 15, 16. The sugar isomerase (SIS) family of enzymes catalyze a wide range of isomerase reactions involved in sugar interconversions 14. In these cases, molecular and quantum mechanics must be used to explain the half-site reactivity 13. However, in some cases of half-site occupancy the conformational changes are too subtle for this definitive change to be observed. Indeed, observation of half occupancy in a crystal structure is considered definitive of enzymes following the sequential model 11. This half-site effect has generally been explained by an allosteric conformational change upon ligand binding that prevents binding at the paired site. These enzymes can then also display positive cooperativity towards the same ligand. Many enzymes consisting of tetramers or higher order multimers are made from two or more pairs of such coupled sites. The extreme form of negative homotropic cooperativity is the so-called “half-site” reactivity, where enzyme active sites are paired, and only one can be active at a time 11, 12. Similarly, both positive and negative cooperativity has been observed in DNA binding proteins 10. This was used as a partial proof for the validity of the “sequential” (Koshland-Némethy-Filmer) model of cooperativity 9. For example, yeast glyceraldehyde-3-phosphate dehydrogenase has long been studied as a model of cooperativity, as it has both positive and negative cooperativity for its substrate NAD + 8. In negatively cooperative enzymes, binding of the first ligand reduces the affinity of binding of subsequent ligands, of particular relevance to ligand binding in signaling networks 7.įurthermore, it is not uncommon for proteins to display both positive and negative cooperativity to the same ligand. Positive cooperativity amplifies the sensitivity of the enzyme’s ligand binding capacity upon increase of ligand concentration and is important for maintaining the responsiveness of biological systems 6. Understanding how cooperativity is manifested in individual enzymes offers the opportunity to further understand the molecular basis of enzyme catalysis and facilitate drug discovery.īoth positive and negative homotropic cooperativity have been observed in a wide range of enzymes 5. Despite the simplicity of the underlying principle, different enzymes seem to adopt different strategies to accomplish cooperative conformational changes. The effects of cooperativity have been characterized structurally in many systems 4. It is a key to link enzymes – the molecular building blocks of life – to the system-level events, such as signaling pathways and cellular responses. Cooperativity critically allows enzymes to make step-like rate responses to substrate concentration changes, and so offer rapid responses to changing cellular conditions. Cooperativity is a fundamental feature of many multi-subunit enzymes 1, 2, 3. ![]()
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