2UU8

X-ray structure of Ni, Ca concanavalin A at Ultra-high resolution (0. 94A)

Summary for 2UU8

• Entry DOI: 10.2210/pdb2uu8/pdb
• Related: 1APN 1BXH 1C57 1CES 1CJP 1CN1 1CON 1CVN 1DQ0 1DQ1 1DQ2 1DQ4 1DQ5 1DQ6 1ENQ 1ENR 1ENS 1GIC 1GKB 1I3H 1JBC 1JN2 1JOJ 1JUI 1JW6 1JYC 1JYI 1NLS 1NXD 1ONA 1QNY 1SCR 1SCS 1TEI 1VAL 1VAM 1VLN 1XQN 2CNA 2CTV 2ENR 3CNA 3ENR 5CNA
• Descriptor: CONCANAVALIN, NICKEL (II) ION, CALCIUM ION, ... (4 entities in total)
• Functional Keywords: lectin, calcium, manganese, metal-binding, ni
• Biological source: CANAVALIA ENSIFORMIS (JACK BEAN)
• Total number of polymer chains: 1
• Total formula weight: 25721.16

Authors

Ahmed, H.U.,Blakeley, M.P.,Cianci, M.,Cruickshank, D.W.J.,Hubbard, J.A.,Helliwell, J.R. (deposition date: 2007-03-01, release date: 2007-07-31, Last modification date: 2023-12-13)

Primary citation

Ahmed, H.U.,Blakeley, M.P.,Cianci, M.,Cruickshank, D.W.J.,Hubbard, J.A.,Helliwell, J.R.
The Determination of Protonation States in Proteins.
Acta Crystallogr.,Sect.D, 63:906-, 2007

PubMed Abstract: The protonation states of aspartic acids and glutamic acids as well as histidine are investigated in four X-ray cases: Ni,Ca concanavalin A at 0.94 A, a thrombin-hirugen binary complex at 1.26 A resolution and two thrombin-hirugen-inhibitor ternary complexes at 1.32 and 1.39 A resolution. The truncation of the Ni,Ca concanavalin A data at various test resolutions between 0.94 and 1.50 A provided a test comparator for the ;unknown' thrombin-hirugen carboxylate bond lengths. The protonation states of aspartic acids and glutamic acids can be determined (on the basis of convincing evidence) even to the modest resolution of 1.20 A as exemplified by our X-ray crystal structure refinements of Ni and Mn concanavalin A and also as indicated in the 1.26 A structure of thrombin, both of which are reported here. The protonation-state indication of an Asp or a Glu is valid provided that the following criteria are met (in order of importance). (i) The acidic residue must have a single occupancy. (ii) Anisotropic refinement at a minimum diffraction resolution of 1.20 A (X-ray data-to-parameter ratio of approximately 3.5:1) is required. (iii) Both of the bond lengths must agree with the expectation (i.e. dictionary values), thus allowing some relaxation of the bond-distance standard uncertainties required to approximately 0.025 A for a '3sigma' determination or approximately 0.04 A for a '2sigma' determination, although some variation of the expected bond-distance values must be allowed according to the microenvironment of the hydrogen of interest. (iv) Although the F(o) - F(c) map peaks are most likely to be unreliable at the resolution range around 1.20 A, if admitted as evidence the peak at the hydrogen position must be greater than or equal to 2.5 sigma and in the correct geometry. (v) The atomic B factors need to be less than 10 A(2) for bond-length differentiation; furthermore, the C=O bond can also be expected to be observed with continuous 2F(o) - F(c) electron density and the C-OH bond with discontinuous electron density provided that the atomic B factors are less than approximately 20 A(2) and the contour level is increased. The final decisive option is to carry out more than one experiment, e.g. multiple X-ray crystallography experiments and ideally neutron crystallography. The complementary technique of neutron protein crystallography has provided evidence of the protonation states of histidine and acidic residues in concanavalin A and also the correct orientations of asparagine and glutamine side chains. Again, the truncation of the neutron data at various test resolutions between 2.5 and 3.0 A, even 3.25 and 3.75 A resolution, examines the limits of the neutron probe. These various studies indicate a widening of the scope of both X-ray and neutron probes in certain circumstances to elucidate the protonation states in proteins.

PubMed: 17642517
DOI: 10.1107/S0907444907029976

Experimental method

X-RAY DIFFRACTION (0.94 Å)