Pairs of protein-protein complexes and the related protein-ligand complexes.

<p>(a) and (b) show trypsin bound to the native protein inhibitor Bovine Pancreatic Trypsin Inhibitor (BPTI) (a, PDB code 1BZX E:I) or to the small-molecule ligand benzamidine (b, 1MBQ A:BEN). Here, the identifiers denote the chains and ligands used. The chain identifiers of the reference proteins are marked in bold. (c) and (d) show insulin growth factor protein bound to an IGF-binding protein (c, 2DSQ I:G) or to a detergent molecule (d, 1GZR B:C15). (e) and (f) show barnase bound to the protein inhibitor barstar (e, 1X1U A:D) or to its natural ligand RNA, in this case the tri-nucleotide GMP (f, 1GOY A:3GP). (g) and (h) display a sugar binding protein and its natural ligand N-acetylglucosamine (h, 1DBN A:NAG) or when it forms a quaternary homomeric structure (g, 2DVG C:B).