Mixed-function protein kinase

Most signaling pathways include steps in which protein kinases are activated. There are three major classes of protein kinases (1) Protein tyrosine kinases catalyze the transfer of the y-phosphate of ATP onto tyrosine residues in their substrate proteins (2) Protein serine/threonine kinases similarly transfer the y-phosphate of ATP onto serine or threonine residues and (3) Mixed-function protein kinases catalyze the transfer of the y-phosphate of ATP onto tyrosine, serine, or threonine residues. 

Tetrahydrobiopterin is not a vitamin, because it can be synthesized from GTP, as shown in Figure 10.2 (Thony et al., 2000). It is the coenzyme for mixed-function oxidases phenylalanine, tyrosine, and tryptophan hydroxylases alkyl glycerol monoxygenase, which catalyzes the cleavage of alkyl glycerol ethers and nitric oxide synthase in the formation of nitric oxide. In addition to its coenzyme role, tetrahydrobiopterin has a direct effect on neurons, acting to stimulate dopamine release via a cAMP-dependent protein kinase and a calcium channel (Koshimura et al., 2000). 

Byus, C. V., Costa, M., Sipes, I. G., Brodie, B. B., and Russell, D. H., Activation of 3 5 -cyclic AMP-dependent protein kinase and induction of ornithine decarboxylase as early events in induction of mixed-function oxygenases, Proc. Natl. Acad. Sci. U.S.A., 73, 1241, 1976. 

---