The characterization of human adenylate kinases 7 and 8 demonstrates differences in kinetic parameters and structural organization among the family of adenylate kinase isoenzymes

Abstract

Differences in expression profiles, substrate specificities, kinetic properties and subcellular localization among the AK (adenylate kinase) isoenzymes have been shown to be important for maintaining a proper adenine nucleotide composition for many different cell functions. In the present study, human AK7 was characterized and its substrate specificity, kinetic properties and subcellular localization determined. In addition, a novel member of the human AK family, with two functional domains, was identified and characterized and assigned the name AK8. AK8 is the second known human AK with two complete and active AK domains within its polypeptide chain, a feature that has previously been shown for AK5. The full-length AK8, as well as its two domains AK8p1 and AK8p2, all showed similar AK enzyme activity. AK7, full-length AK8, AK8p1 and AK8p2 phosphorylated AMP, CMP, dAMP and dCMP with ATP as the phosphate donor, and also AMP, CMP and dCMP with GTP as the phosphate donor. Both AK7 and full-length AK8 showed highest affinity for AMP with ATP as the phosphate donor, and proved to be more efficient in AMP phosphorylation as compared with the major cytosolic isoform AK1. Expression of the proteins fused with green fluorescent protein demonstrated a cytosolic localization for both AK7 and AK8.

Differential Expression of Adenine Nucleotide Converting Enzymes in Mitochondrial Intermembrane Space: A Potential Role of Adenylate Kinase Isozyme 2 in Neutrophil Differentiation

“…The AK family comprises nine isozymes that are located in various subcellular compartments and are distributed in tissues as follows: AK1, 5, 7, and 8 are located in the cytosol, AK2, 3, and 4 are located in the mitochondria, and AK6 is located in the nucleus [1], [2], [10]–[12]. In addition, a recent study using GFP-fusion proteins demonstrated AK9 in both cytosolic and nuclear compartments [13].…”

Developmental Enhancement of Adenylate Kinase-AMPK Metabolic Signaling Axis Supports Stem Cell Cardiac Differentiation

“…Indeed, a lower ratio of p-AMPK/AMPK was detected in the cardiomyocyte nucleus after AK siRNA treatment, which dropped from an average of 3.0 in control to 1.5 in cardiomyocytes isolated from AK1/AK2/AK5-knockdown embryoid bodies. Of note, AK1 knockout mice present no apparent developmental cardiac defects which could be related to overlapping function and compensation provided by complementary adenylate kinase isoforms of which eight are currently known [7], [64]. Deletion of AK2, an isoform silenced here, is embryonically lethal in mice [65] and Drosophila [29] with severe defects in mitochondrial structure and energetics, and human mutations of AK2 are associated with developmental defects [30], [31].…”

Congenital Hydrocephalus in Genetically Engineered Mice

“…Situs inversus was not detected in Ak7 -/-mice. Ak8 (adenylate kinase 8): 71 All homozygous Ak8 -/-mice in this line developed mild to moderate hydrocephalus, usually restricted to the lateral ventricles (Fig. 13).…”

“…It has been postulated that AK7 and AK8 activities may be crucial to meeting the high-energy requirements of motile cilia. 71 Although mice deficient in either AK7 or AK8 each developed hydrocephalus, they differed markedly in disease severity and age of onset. Decreased viability was clearly evident in Ak7 -/-mice, which showed reduced number of homozygotes at time of weaning with very few mutant mice surviving beyond 3 weeks of age.…”