Pantothenate kinase catalyzes the key regulatory step in CoA biosynthesis in bacteria and mammals (1-3). In mammals, there are four distinct kinases that exhibit tissue-specific expression as follows: PANK1␣ and PANK1 are splice variants of the PANK1 gene, and PANK2 and PANK3 are encoded by the PANK2 and PANK3 genes, respectively (4, 5). The kinase isoforms possess almost identical catalytic cores but differ in their amino termini that direct the enzymes to different subcellular compartments (6). Isoform 1␣ is targeted to the nucleus, whereas isoform 1 is associated with endosomes in humans and mice. Isoform 2 of mice is cytosolic, but the PANK2 gene in humans encodes a protein with both nuclear localization and mitochondrial targeting sequences. Isoform 3 is cytosolic in both species. All pantothenate kinases operate by a compulsory ordered mechanism with ATP⅐Mg 2ϩ as the leading substrate followed by pantothenate ( Fig. 1) (7). The principal mechanism for controlling mammalian pantothenate kinase activity is through feedback inhibition by acetyl-CoA, and the isoforms differ in their sensitivity to this regulator ( Fig. 1) (4, 8, 9). The 1␣ and 1 isoforms are least sensitive to inhibition, whereas isoforms 2 and 3 are more potently inhibited by acetyl-CoA. Acetyl-CoA inhibition is competitive with ATP⅐Mg 2ϩ , but acetyl-CoA binds far more tightly than ATP (10). Acyl-carnitines antagonize the inhibition of pantothenate kinases by acetyl-CoA (8).The importance of pantothenate kinase to mammalian physiology is highlighted by the phenotypes of knock-out mice and their connection to human disease. Isoform 1 is most highly expressed in the liver, and Pank1 Ϫ/Ϫ mice have lower total hepatic CoA and exhibit fatty acid -oxidation and glucose homeostasis defects in the fasted state (11). In addition, Pank1 Ϫ/Ϫ Lep Ϫ/Ϫ mice have dramatically lower blood glucose and insulin levels compared with their diabetic Lep Ϫ/Ϫ counterparts, which highlights the connection between isoform 1 and glucose homeostasis (12). Consistent with this, an association between polymorphisms in the PANK1 gene and insulin levels was uncovered in a cohort of individuals in Finland (13). It has also been shown that the severe neurodegenerative disease pantothenate kinase-associated neurodegeneration arises from mutations in the human PANK2 gene (14). Unfortunately, Pank2 Ϫ/Ϫ mice do not recapitulate the pantothenate kinaseassociated neurodegeneration disease phenotype (15,16), and this may be due either to differences in the levels of isoform expression in mouse and human brains or to the fact that human PANK2 accumulates in the intra-membrane space in mitochondria, whereas mouse isoform 2 is cytosolic (9). Finally, Pank1 Ϫ/Ϫ Pank2 Ϫ/Ϫ double knock-out mice are unable to metabolize fats and ketones resulting in early postnatal death (16), and Pank1 Ϫ/Ϫ Pank3 Ϫ/Ϫ and Pank2 Ϫ/Ϫ Pank3 Ϫ/Ϫ double knock-out mice are both embryonic lethal. A chemical knockout of all pantothenate kinases in adult mice resulted in an 80% reduction in hepatic CoA levels a...