Catalyzes the reversible transfer of the terminal phosphate group between ATP and AMP. Plays an important role in cellular energy homeostasis and in adenine nucleotide metabolism. AMP + ATP = 2 ADP Optimum temperature is 35 degrees Celsius. Thermal denaturation midpoint (Tm) is 43.3 degrees Celsius and is raised to 60.4 degrees Celsius when AK is complexed with the inhibitor Ap5A. Purine metabolism; AMP biosynthesis via salvage pathway; AMP from ADP: step 1/1. Monomer. Consists of three domains, a large central CORE domain and two small peripheral domains, NMPbind and LID, which undergo movements during catalysis. The LID domain closes over the site of phosphoryl transfer upon ATP binding. Assembling and dissambling the active center during each catalytic cycle provides an effective means to prevent ATP hydrolysis. Some bacteria have evolved a zinc-coordinating structure that stabilizes the LID domain. Belongs to the adenylate kinase family.