| Both hexokinase 1 and hexokinase 2 incorporate ;Hexokinase 2 was also phosphorylated in vivo to some extent under repressed growth conditions. The possibility that, for hexokinase 2, the phosphorylation involved a cAMP-dependent protein kinase was suggested by the presence of an Arg-Arg-Leu-Ser consensus phosphorylation sequence. The enzyme was indeed phosphorylated in vitro by the cAMP protein kinase. However, mutant enzymes altered in the sequence were still phosphorylated in vitro and, in vivo growth, glucose transport, and catabolite repression were also not observably altered. Furthermore, in vivo labelling experiments in strains altered in the cAMP effector pathway gave no support for a direct role of the cAMP protein kinase in phosphorylation of hexokinase. However, an indirect role of cAMP protein kinase activity somehow diminishing hexokinase phosphorylation in vivo was implied by the results. |
