And so it's the p loop structure found inside this domain of the enzyme that actually is responsible for binding and interacting with this substrate molecule, the triphosphate group of that particular nucleotide triphosphate. So in the case of adenylate kinase, it's the triphosphate group of the ATP that the p loop actually interacts with. Now, the second thing you have to know about nucleoside monophosphate kinases is they use a specific mechanism of enzyme catalysis known as metal ion catalysis. Now, we actually discussed metal ion catalysis previously when we discussed proteases and carbonic and hydrates. But the major difference between that type of metaline catalysis and the metaline catalysis that takes place within an MP kinases is here that metal ion doesn't actually interact with the active sites enzyme, but it interacts with the substrate molecule itself. So we see that NMP kinases require the presence of a metal atom such as magnesium or a manganese.