Daniel E. Holloway

Crystal Structures of Oligomeric Forms of the Ip-10/Cxcl10 Chemokine

We have determined the structure of wild-type IP-10 from three crystal forms. The crystals provide eight separate models of the IP-10 chain, all differing substantially from a monomeric IP-10 variant examined previously by NMR spectroscopy. In each crystal form, IP-10 chains form conventional beta sheet dimers, which, in turn, form a distinct tetrameric assembly. The M form tetramer is reminiscent of platelet factor 4, whereas the T and H forms feature a novel twelve-stranded beta sheet. Analytical ultracentrifugation indicates that, in free solution, IP-10 exists in a monomer-dimer equilibrium with a dissociation constant of 9 microM. We propose that the tetrameric structures may represent species promoted by the binding of glycosaminoglycans. The binding sites for several IP-10-neutralizing mAbs have also been mapped.

Ribonuclease A Homologues of the Zebrafish: Polymorphism, Crystal Structures of Two Representatives and Their Evolutionary Implications.

The widespread and functionally varied members of the ribonuclease A (RNase A) superfamily provide an excellent opportunity to study evolutionary forces at work on a conserved protein scaffold. Representatives from the zebrafish are of particular interest as the evolutionary distance from non-ichthyic homologues is large. We conducted an exhaustive survey of available zebrafish DNA sequences and found significant polymorphism among its four known homologues. In an extension of previous nomenclature, the variants have been named RNases ZF-1a–c,-2a–d,-3a–e and-4. We present the first X-ray crystal structures of zebrafish ribonucleases, RNases ZF-1a and-3e at 1.35-and 1.85 Å resolution, respectively. Structure-based clustering with ten other ribonuclease structures indicates greatest similarity to mammalian angiogenins and amphibian ribonucleases, and supports the view that all present-day ribonucleases evolved from a progenitor with three disulphide bonds. In their details, the two structures are intriguing melting-pots of features present in ribonucleases from other vertebrate classes. Whereas in RNase ZF-1a the active site is obstructed by the C-terminal segment (as observed in angiogenin), in RNase ZF-3e the same region is open (as observed in more catalytically efficient homologues). The progenitor of present-day ribonucleases is more likely to have had an obstructive C terminus, and the relatively high similarity (late divergence) of RNases ZF-1 and-3 infers that the active site unblocking event has happened independently in different vertebrate lineages.

Structure of Murine Angiogenin: Features of the Substrate- and Cell-Binding Regions and Prospects for Inhibitor-Binding Studies.

Angiogenin is an unusual member of the pancreatic ribonuclease superfamily that induces blood-vessel formation and is a promising anticancer target. The three-dimensional structure of murine angiogenin (mAng) has been determined by X-ray crystallography. Two structures are presented: one is a complex with sulfate ions (1.5 Angstroms resolution) and the other a complex with phosphate ions (1.6 Angstroms resolution). Residues forming the putative B(1), P(1) and B(2) subsites occupy positions similar to their hAng counterparts and are likely to play similar roles. The anions occupy the P(1) subsite, sulfate binding conventionally and phosphate adopting two orientations, one of which is novel. The B(1) subsite is obstructed by Glu116 and Phe119, with the latter assuming a less invasive position than its hAng counterpart. Hydrophobic interactions between the C-terminal segment and the main body of the protein are more extensive than in hAng and may underly the lower enzymatic activity of the murine protein. Elsewhere, the structure of the H3-B2 loop supports the view that hAng Asn61 interacts directly with cell-surface molecules and does not merely stabilize adjacent regions of the hAng structure. mAng crystals appear to offer small-molecule inhibitors a clear route to the active site and may even withstand a reorientation of the C-terminal segment that provides access to the cryptic B(1) subsite. These features represent considerable advantages over crystalline hAng and bAng.

C3 exoenzyme from Clostridium botulinum: structure of a tetragonal crystal form and a reassessment of NAD-induced flexure

C3 exoenzyme from Clostridium botulinum (C3bot1) ADP-ribosylates and thereby inactivates Rho A, B and C GTPases in mammalian cells. The structure of a tetragonal crystal form has been determined by molecular replacement and refined to 1.89 A resolution. It is very similar to the apo structures determined previously from two different monoclinic crystal forms. An objective reassessment of available apo and nucleotide-bound C3bot1 structures indicates that, contrary to a previous report, the protein possesses a rigid core formed largely of beta-strands and that the general flexure that accompanies NAD binding is concentrated in two peripheral lobes. Tetragonal crystals disintegrate in the presence of NAD, most likely because of disruption of essential crystal contacts.

Crystal structure of Onconase at 1.1 Å resolution--insights into substrate binding and collective motion.

Onconase® (ONC) is an amphibian member of the pancreatic ribonuclease superfamily that is selectively toxic to tumor cells. It is a much less efficient enzyme than the archetypal ribonuclease A and, in an attempt to gain further insight, we report the first atomic resolution crystal structure of ONC, determined in complex with sulfate ions at 100 K. The electron density map is of a quality sufficient to reveal significant nonplanarity in several peptide bonds. The majority of active site residues are very well defined, with the exceptions being Lys31 from the catalytic triad and Lys33 from the B1 subsite, which are relatively mobile but rigidify upon nucleotide binding. Cryocooling causes a compaction of the unit cell and the protein contained within. This is principally the result of an inward movement of one of the lobes of the enzyme (lobe 2), which also narrows the active site cleft. Binding a nucleotide in place of sulfate is associated with an approximately perpendicular movement of lobe 2 and has little further effect on the cleft width. Aspects of this deformation are present in the principal axes of anisotropy extracted from Cα atomic displacement parameters, indicating its intrinsic nature. The three lowest‐frequency modes of ONC motion predicted by an anisotropic network model are compaction/expansion variations in which lobe 2 is the prime mover. Two of these have high similarity to the cryocooling response and imply that the essential ‘breathing’ motion of ribonuclease A is conserved in ONC. Instead, shifts in conformational equilibria may contribute to the reduced ribonucleolytic activity of ONC.

Influence of Naturally-Occurring 5'-Pyrophosphate-Linked Substituents on the Binding of Adenylic Inhibitors to Ribonuclease A: An X-Ray Crystallographic Study.

Ribonuclease A is the archetype of a functionally diverse superfamily of vertebrate‐specific ribonucleases. Inhibitors of its action have potential use in the elucidation of the in vivo roles of these enzymes and in the treatment of pathologies associated therewith. Derivatives of adenosine 5′‐pyrophosphate are the most potent nucleotide‐based inhibitors known. Here, we use X‐ray crystallography to visualize the binding of four naturally‐occurring derivatives that contain 5′‐pyrophosphate‐linked extensions. 5′‐ATP binds with the adenine occupying the B2 subsite in the manner of an RNA substrate but with the γ‐phosphate at the P1 subsite. Diadenosine triphosphate (Ap3A) binds with the adenine in syn conformation, the β‐phosphate as the principal P1 subsite ligand and without order beyond the γ‐phosphate. NADPH and NADP+ bind with the adenine stacked against an alternative rotamer of His119, the 2′‐phosphate at the P1 subsite, and without order beyond the 5′‐α‐phosphate. We also present the structure of the complex formed with pyrophosphate ion. The structural data enable existing kinetic data on the binding of these compounds to a variety of ribonucleases to be rationalized and suggest that as the complexity of the 5′‐linked extension increases, the need to avoid unfavorable contacts places limitations on the number of possible binding modes.

Crystal structures of murine angiogenin-2 and -3-probing 'structure--function' relationships amongst angiogenin homologues.

Angiogenin (Ang) is a potent inducer of neovascularization. Point mutations in human Ang have been linked to cancer progression and two neurodegenerative diseases: amyotrophic lateral sclerosis and Parkinsons disease. Intensive structural and functional analyses of Ang have been paramount in assigning functions to this novel homologue of bovine pancreatic RNase A. However, inhibitor‐binding studies with crystalline Ang (for designing potential anti‐cancer drugs) have been hampered as a result of the inaccessibility of the active site. Experiments with the murine homologues of Ang have not only overcome the obvious practical limitations encountered when studying the role of a human protein in healthy individuals, but also the crystal structures of murine angiogenins (mAng and mAng‐4) have revealed themselves to have greater potential for the visualization of small‐molecule inhibitor binding at the active site. In the present study, we report the crystal structures of two more murine Ang paralogues, mAng‐2 and mAng‐3, at 1.6 and 1.8 Å resolution, respectively. These constitute the first crystal structures of an Ang with a zinc ion bound at the active site and provide some insight into the possible mode of inhibition of the ribonucleolytic activity of the enzyme by these divalent cations. Both structures show that the residues forming the putative P1, B1 and B2 subsites occupy positions similar to their counterparts in human Ang and are likely to have conserved roles. However, a less obtrusive conformation of the C‐terminal segment in mAng‐3 and the presence of a sulfate ion in the B1 subsite of mAng‐2 suggest that these proteins have the potential to be used for inhibitor‐binding studies. We also discuss the biological relevance of the structural similarities and differences between the different Ang homologues.

Adenosylcobalamin-Dependent Enzymes

This chapter describes recent developments in our understanding of adenosylcobalamin1 (AdoCbl, coenzyme B12)-dependent enzymes. We will focus on the ten B12 enzymes known that catalyze unusual rearrangement reactions. These reactions involve the interchange of an electronwithdrawing group, X, on one carbon with a hydrogen atom on an adjacent carbon, as shown in Figure 1. This review will place greatest emphasis on those enzymes that catalyze carbon skeleton rearrangements, for which a significant amount of new structural and mechanistic information is now emerging. The eleventh AdoCbl-dependent enzyme, class II ribonucleotide reductase, will be discussed only in relation to the function of the AdoCbl coenzyme, which serves the same basic role as in the