Kinase Family PDHK
The Pyruvate Dehydrogenase Kinases are mitochondrial metabolic regulators. They phosphorylate and inactivate the E1 subunits of pyruvate dehydrogenase, and so inhibit the citric acid (TCA, Krebs) cycle. They are related to the histidine kinases found in bacteria and eukaryotes, but phosphorylate on serine rather than histidine/aspartate, and do not seem to autophosphorylate on histidine Steussy.
PDHK is found in most eukaryotic lineages (notably missing only in organisms with reduced or absent mitochondria, such as Giardia, Trichomonas and Entamoeba). The major subfamily is also called PDHK, while many holozoans have a distinctive single-copy BCKDK (Branched-Chain Ketoacid Dehydrogenase Kinase) subfamily gene. Fungi and some other organisms have additional members that are difficult to classify.
The PDHK gene is single copy in most animals, but has four copies in vertebrates, with different tissue distribution and sensitivity to regulators Korotchkina. BCKDK is always single-copy.
All PDHKs have a C-terminal Histidine Kinase-like domain, and an N-terminal BCDHK_Adom3 domain (despite the name it is not specific for BCKDKs).
PDHK has only one known substrate, the E1 (or alpha) subunits of Pyruvate Dehydrogenase (PDH). PDH transforms pyruvate (from glycolysis) into acetyl coA (which feeds the citric acid cycle in the mitochondrion), so phosphorylation of PDH blocks the citric acid cycle and results in pyruvate being transformed to lactate instead.
BCKDK inactivates BCKDH, by phosphorylation of the homologous E1 subunit. The BCKDH complex is biochemically similar to the PDH complex, but is involved in the degradation of branched-chain amino acids (valine, leucine, isoleucine).
Substrates and mechanism
Mammalian PDH is phosphorylated on three serines by PDHK. All four mammalian PDHKs can phosphorylate Site 1 (S293 in human) and Site 2 (S300), while only PDK1 was seen to phosphorylate Site 1 (S230) Korotchkina. Accordingly, site 3 is poorly conserved outside of vertebrates, while Site 1 is absolutely conserved (both in PDH and BCKDH) and Site 2 is largely well conserved within PDH.
Mammalian BCKDHA is phosphorylated on two serines: one is equivalent to the conserved Site 1 of PDH (S337 in human), while the other (Site 2b) is 3 AA downstream of site 2 (S347 in human) and likely to be equivalent. Site 1 is perfectly conserved, while Site 2b is well conserved. In nematodes and most insects, where BCKDK has been lost, some still retain these sites,
BCKDK has also been reported to phosphorylate S292 on BCKDHB, a second subunit of the BCKDH complex, and so inactivate it. Wynn
Both BCKDK and PDHK have been reported to autophosphorylate. The rat autophosphorylation site was tentatively mapped to S52 Davie, a position conserved in most bilaterian BCKDK genes, but not in PDHKs.
PDHK is a target for diabetes therapies; inhibition of PDHK should increase glucose uptake and metabolism. Mayers
Mutations in several subunits of BCKDH result in Maple Syrup Urine Disease (OMIM), though mutations have not been found in BCKDK.
- Davie pmid=7649998
- Korotchkina pmid=11486000
- Mayers pmid=15787608
- Steussy pmid=11483605
- Wynn pmid=15576032