Journal of Molecular Biology
Regular articleA novel main-chain anion-binding site in proteins: the nest. A particular combination of φ,ψ values in successive residues gives rise to anion-binding sites that occur commonly and are found often at functionally important regions1
Introduction
In spite of much work1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 describing small, mainly hydrogen-bonded, motifs of under ten residues in proteins, it may seem surprising to come across common and functionally useful protein features of this sort recurring in a variety of situations that have remained essentially unrecognised. Individual types are known and even familiar, but the structural similarities and relationships between them have not, as far as we can tell, been described hitherto.
Here, we regard carbonyl groups as anionic because of their partial charge. In relation to anion binding sites, carbonyl oxygen atoms belonging to amides behave similarly to those of carboxylates. A well-known situation where main-chain NH groups form an anion-binding site is at the N terminus of an α-helix. Because their NH groups all point to the C terminus, most α-helices have two or three free NH groups not hydrogen-bonded to main-chain CO groups and these often form anion-binding sites of various sorts. They may bind carbonyl groups for example,14, 21 or phosphate groups.23 They sometimes occur on their own without forming a proper α-helix (residues 27-29 of phospholipases A2,24 bound to the carbonyl group of the conserved aspartate 43, form one), identifiable by the αR or γRdihedral angles of a minimum of two successive residues. However, these motifs are not the primary concern of the present work.
We point to another feature where main-chain NH groups form a potential anion-binding site. It is characterized by alternating residues with γR(or αR) and γL(or αL) main-chain dihedral angles in either order. A consequence of two such adjacent residues is often the formation of an atom-sized concavity with their two main-chain NH groups, plus the NH of the amino acid following, pointing inwards, ideal for an anion-binding site, as seen in Figure 1(a). The first and third NH groups are well positioned for hydrogen bonding to an atom in the cavity generated, while the middle NH sometimes also does so though it points more to the side. We propose the word nest as a name for this three NH motif.
Just as α-helices in native proteins can be right-handed or, rarely, left-handed, so nests may exist as one of two enantiomers, this form of enantiomerism being restricted to the main-chain atoms. We use the terminology RL and LR to refer to them. RL is where the first residue is αR or γR and the second αL or γL. If the residues are the other way round the nest is of type LR. Both are found, although RL nests are more common. However, the oxyanion hole in serine proteases is an LR nest.
This survey reveals that compound nests of the form RLR, and tandem nests of the form RLLR, are found in proteins and are prominent in certain functional regions. In the compound type, which is commoner, the two overlapping nests each have a potential binding site for an anionic atom facing in about the same direction, such that they form a wider concavity that may accommodate anionic groups rather than just single atoms. Long compound nests we have found occur in the P-loops of nucleotide triphosphate-binding proteins (LRLR, with five consecutive NH groups facing into the concavity) and in the loops that bind [2Fe2S] or [4Fe4S] in iron-sulfur proteins (RLRLR in some ferredoxins, with six such NH groups).
Section snippets
Why nests constitute a novel class of motif
We define nests from the φ,ψ angles of successive residues given in Methods. Essentially, one set is in the γR or αR region and one is γL or αL. The distribution of residues in proteins in the Ramachandran plot is skewed towards the αR or αL, rather than the γR or γL, regions both in terms of the distribution observed and energetic conformational favorability. Yet, in the motifs found by these criteria, there are relatively few αRαL or αLαRcombinations. This appears to be related to the
Conclusion
The nest, the name we give to the depression formed by the main-chain NH groups of three consecutive amino acid residues in a particular conformation, is a common means by which main-chain atoms of proteins bind anions for various purposes. The paperclip/Schellman loop is one of the commonest features exhibiting a nest; it can be regarded as a means of stabilizing the carbonyl groups at α-helical C termini. Such nests bind two oxygen atoms, whereas others bind only one. Nests occur in two
Methods
The primary database, used as the basis for the work described first (Figure 1, Figure 2, Figure 3, Figure 4, Table 1, Table 2, Table 3, Table 4), is a set of 67 protein structures, influenced by the ‘PDB_SELECT’ list at ftp://ftp.embl-heidelberg.de/pub/databases/pdb_select.51 All have less than 25 % sequence identity with each other, are refined by X-ray diffraction at a resolution of less than 1.5 Å, and have an R-factor less than 20 %. The PDB codes, with the subunit letter added when
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Edited by J. Thornton