Thymidine kinase is one of the many nucleoside kinases that are responsible for nucleoside phosphorylation. GAPDH is activated via tyrosine phosphorylation by Src. Protein phosphatase 1 counteracts phosphorylation of S695. Normal cells primarily produce energy through mitochondrial oxidative phosphorylation. Some phosphorylation of p70 S6 kinase was discovered. Salicylate toxicity works by uncoupling oxidative phosphorylation. UCPs are transmembrane proteins that decrease the proton gradient generated in oxidative phosphorylation. Like kinases, phosphatases too play a role in regulating the phosphorylation of tau. This prevents the phosphorylation and degradation of cyclin D1. These modifications include acetylation, methylation, ubiquitylation, phosphorylation, sumoylation, ribosylation and citrullination. Elucidating complex signaling pathway phosphorylation events can be difficult. Electrochemical gradients also play a role in establishing proton gradients in oxidative phosphorylation in mitochondria. ATP synthase then ensues exactly as in oxidative phosphorylation. Deletions or mutations to segments of mtDNA lead to defective function of oxidative phosphorylation. Not all inhibitors of oxidative phosphorylation are toxins.