Overall structure and active site of <i>Pf</i>SpdS.

Abstract

<p>(A) Monomer architecture. The N-terminal beta-sheet domain is light blue; the C-terminal Rossmann fold domain is dark blue. The active site is indicated in a cleft between the two domains, marked by a stick model of bound MTA and putrescine (green, based on PDB ID: 4BP1). The gatekeeper loop spanning the active site is shown in orange; when ligands are bound it adopts a loop-3₁₀ helix-loop structure that is approximated in the representation shown. (B) Active site. Labeled yellow shaded or outlined oval shapes of different sizes represent the indicated parts of the active site referred to in the text. The larger dcAdoMet site (black-outlined oval at right) is conceptually divided into an MTA cavity (large upper shaded oval) and a central aminopropyl cavity (small lower shaded oval). The putrescine site (central black-outlined oval) is adjacent to a distal aminopropyl cavity (shaded oval at upper left). The substrates dcAdoMet and putrescine are represented as stick cartoons with green carbon atoms and other atoms in atomic colors (blue nitrogen, red oxygen, and yellow sulfur); these substrates do not occur together in any existing crystal structure because the enzyme reaction would occur. The cartoon is a composite based on separate structures with dcAdoMet and with MTA and putrescine.</p