The hyperthermophilic bacterium Thermotoga neapolitana possesses two isozymes of the 3-phosphoglycerate kinase/triosephosphate isomerase fusion protein.

The phosphoglycerate kinase (pgk), triosephosphate isomerase (tpi), and enolase (eno) genes from Thermotoga neapolitana have been cloned and expressed in Escherichia coli. In high copy number, the pgk gene complemented an E. coli pgk- strain. In T. neapolitana, the pgk and tpi genes appear to be fused and eno is near those genes. Like T. maritima, T. neapolitaná produces phosphoglycerate kinase as both an individual enzyme and a fusion protein with triosephosphate isomerase, and triosephosphate isomerase activity is not found without associated phosphoglycerate kinase activity. Unlike T. maritima, which forms only a 70-kDa fusion protein, T. neapolitana expresses both 73-kDa and 81-kDa isozymes of this fusion protein. These isozymes are present in both T. neapolitana cells and in E. coli cells expressing T. neapolitana genes.