text: To date there have been nine human ADK protein isoforms identified.  While some of these are ubiquitous throughout the body, some are localized into specific tissues.  For example, ADK7 and ADK8 are both only found in the cytosol of cells; and ADK7 is found in skeletal muscle whereas ADK8 is not.  Not only do the locations of the various isoforms within the cell vary, but the binding of substrate to the enzyme and kinetics of the phosphoryl transfer are different as well.  ADK1, the most abundant cytosolic ADK isozyme, has a K about a thousand times higher than the K of ADK7 and 8, indicating a much weaker binding of ADK1 to AMP. Sub-cellular localization of the ADK enzymes is done by including a targeting sequence in the protein.  Each isoform also has different preference for NTP's.  Some will only use ATP, whereas others will accept GTP, UTP, and CTP as the phosphoryl carrier.
  Some of these isoforms prefer other NTP's entirely.  There is a mitochondrial GTP:AMP phosphotransferase, also specific for the phosphorylation of AMP, that can only use GTP or ITP as the phosphoryl donor. ADK has also been identified in different bacterial species and in yeast. Two further enzymes are known to be related to the ADK family, i.e. yeast uridine monophosphokinase and slime mold UMP-CMP kinase. Some residues are conserved across these isoforms, indicating how essential they are for catalysis. One of the most conserved areas includes an Arg residue, whose modification inactivates the enzyme, together with an Asp that resides in the catalytic cleft of the enzyme and participates in a salt bridge.