P-loop

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A P-loop or phosphate-binding loop, is an ATP/GTP binding site motif found in many nucleotide-binding proteins. It is a flexible loop between the first β-strand and the first α-helix of the protein and can interact with one of the phosphate groups of the nucleotide. Typically it has the amino acid sequence Ala/Gly-X-X-X-X-Gly-Lys-Ser/Thr. Nucleotide-binding proteins share several more or less conserved sequence motifs; the P-loop is the best conserved one of these. It is widely spread, probably because of its ability to help the protein to undergo great conformational changes upon nucleotide binding and hydrolysis.

Some proteins bind nucleotides in a completely different way and have no p-loop, examples include glycolytic kinases and E1-E2 ATPases.

Some proteins with P-loops include ATP synthase, myosin, transducin, helicase and G-proteins.

PTPs (Protein tyrosine phosphatases) which catalyse the hydrolysis of an inorganic phosphate from a tyrosine residue also have a P-loop. The conserved sequence of this motif is a cysteine residues, followed by any five residues then an arginine with either a serine or a threonine following. In all it is eight amino acid residues in length, and given the single letter amino acid code CX5R(S/T), where X is any residue, C is a cysteine, R an arginine and S/T is a serine/threonine residue.