The biochemical pathway that makes BH4 requires GTP as the substrate and three dedicated enzymes (GTP cyclohydrolase I, 6pyruvoyltetrahydropterin synthase, and sepiapterin reductase); the key redox cofactors used in the pathway and Item Scheme showing the enzymatic reactions that catalyze the pyruvate dehydrogenase complex, including pyruvate dehydrogenase activity and diaphorase activity of DLD (A) as well as the glutathione reductase peroxidase activity (B). Item Scheme showing the enzymatic reactions that catalyze the pyruvate dehydrogenase complex, including pyruvate dehydrogenase activity and diaphorase activity of DLD (A) as well as the glutathione reductase peroxidase activity (B). Expression and enzyme activity of glutathione reductase is upregulated by Fe deficiency in graminaceous plants Plant Molecular Biology Springer Nature Link