P-loop

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Alignment of the H-Ras mutant A59G mutants in complex with GppNHp (green cartoon) and GDP (cyan cartoon). The P-loop main chain is shown in red, the Mg²⁺ ion as green sphere and the side chains of the amino acids K16 and S17 are shown as sticks.

A P-loop or phosphate-binding loop, is an ATP/GTP binding site motif found in many nucleotide-binding proteins. It is a glycine-rich loop preceded by a β-sheet and followed by an α-helix. It interacts with the phosphate groups of the nucleotide and the Mg²⁺ ion, which coordinates the β- and the γ-phosphate.

The consensus sequence of the P-loop is GxxxxGK(S/T), which is often referred to as Walker A motif. Upon nucleotide hydrolysis the P-loop does not change its conformation significantly, it rather stays bound to the remaining phosphate groups.

Proteins with (functional) P-loops include ATP synthase, myosin, transducin, helicase and Small GTPases like Ras.

PTPs (Protein tyrosine phosphatases) which catalyse the hydrolysis of an inorganic phosphate from a phosphotyrosine residue (see Tyrosine kinase) also contain a P-loop-similar motif. The conserved sequence of this motif is CxxxxxR(S/T)