Backbone 1H, 13C, and 15N NMR assignments were obtained for the complex of chicken muscle adenylate kinase (AK) with its bisubstrate analog, MgAP5A [magnesium P1,P5-bis(5'-adenosyl)-pentaphosphate]. The assignments were used to elucidate the secondary structures and the enzyme-MgAP5A interactions. The work involves two unusual features: the molecular weight of AK (21.6 kDa) is one of the largest, on a monomeric basis, for which nearly complete assignment has been reported to date, and the assignment was performed at pH 7.1 instead of the acidic pH used for most other proteins. The results are summarized as follows. Firstly, unambiguous sequential assignments of backbone resonances have been achieved effectively by the combined use of two sequential assignment methods: NOE-directed assignments and the recently developed 1J-coupling-directed assignments. The starting points of the assignments were provided by several specifically labeled enzyme samples. Over 90% of the backbone 1H, 13C, and 15N resonances have been assigned. Secondly, spin system information was obtained from the HCCH-TOCSY and HCCH-COSY experiments as well as from 2D homonuclear NMR data. Overall, the side-chain resonances of ca. 40% of the residues, including most of the those displaying NOEs with the adenosine moieties of MgAP5A, have been assigned. Thirdly, secondary structural elements in the AK-MgAP5A complex were identified by extensive analyses of 1H-15N 2D HMQC-NOESY and 3D NOESY-HMQC spectra. Overall, the enzyme consists of ca. 60% alpha-helices and a five-stranded parallel beta-sheet. The results are compared with the secondary structure of the free AK from porcine muscle in crystals [Dreusicke, D., Karplus, P. A., & Schulz, G. E. (1988) J. Mol. Biol. 199, 359-371]. Lastly, most of the intermolecular NOEs between AK and the adenosine moieties of MgAP5A have been identified: Thr39, Leu43, Gly64, Leu66, Val67, Val72, and Gln101 are in proximity to the adenosine moiety of the adenosine 5'-monophosphate site, whereas Thr23 is in proximity to that of the adenosine 5'-triphosphate site. These data are discussed in relation to previous results from site-directed mutagenesis, NMR, and X-ray studies and in relation to the mechanism of catalysis.