Identification of structural determinants for inhibition strength and specificity of wheat xylanase inhibitors TAXI-IA and TAXI-IIA

Triticum aestivum xylanase inhibitor ( TAXI)- type inhibitors are active against microbial xylanases from glycoside hydrolase family 11, but the inhibition strength and the specificity towards different xylanases differ between TAXI isoforms. Mutational and biochemical analyses of TAXI- I, TAXI- IIA and Bacillus subtilis xylanase A showed that inhibition strength and specificity depend on the identity of only a few key residues of inhibitor and xylanase [ Fierens K et al. ( 2005) FEBS J 272, 5872 - 5882; Raedschelders G et al. ( 2005) Biochem Biophys Res Commun 335, 512 - 522; Sorensen JF & Sibbesen O ( 2006) Protein Eng Des Sel 19, 205 - 210; Bourgois TM et al. ( 2007) J Biotechnol 130, 95 - 105]. Crystallographic analysis of the structures of TAXI- IA and TAXI- IIA in complex with glycoside hydrolase family 11 B. subtilis xylanase A now provides a substantial explanation for these observations and a detailed insight into the structural determinants for inhibition strength and specificity. Structures of the xylanase - inhibitor complexes show that inhibition is established by loop interactions with active- site residues and substrate- mimicking contacts in the binding subsites. The interaction of residues Leu292 of TAXI- IA and Pro294 of TAXI- IIA with the -2 glycon subsite of the xylanase is shown to be critical for both inhibition strength and specificity. Also, detailed analysis of the interaction interfaces of the complexes illustrates that the inhibition strength of TAXI is related to the presence of an aspartate or asparagine residue adjacent to the acid / base catalyst of the xylanase, and therefore to the pH optimum of the xylanase. The lower the pH optimum of the xylanase, the stronger will be the interaction between enzyme and inhibitor, and the stronger the resulting inhibition.status: publishe

How calcium inhibits the magnesium-dependent enzyme human phosphoserine phosphatase

The structure of the Mg(2+)-dependent enzyme human phosphoserine phosphatase (HPSP) was exploited to examine the structural and functional role of the divalent cation in the active site of phosphatases. Most interesting is the biochemical observation that a Ca(2+) ion inhibits the activity of HPSP, even in the presence of added Mg(2+). The sixfold coordinated Mg(2+) ion present in the active site of HPSP under normal physiological conditions, was replaced by a Ca(2+) ion by using a crystallization condition with high concentration of CaCl(2) (0.7 m). The resulting HPSP structure now shows a sevenfold coordinated Ca(2+) ion in the active site that might explain the inhibitory effect of Ca(2+) on the enzyme. Indeed, the Ca(2+) ion in the active site captures both side-chain oxygen atoms of the catalytic Asp20 as a ligand, while a Mg(2+) ion ligates only one oxygen atom of this Asp residue. The bidentate character of Asp20 towards Ca(2+) hampers the nucleophilic attack of one of the Asp20 side chain oxygen atoms on the phosphorus atom of the substrate phosphoserine

Structural basis for inhibition of Aspergillus niger xylanase by triticum aestivum xylanase inhibitor-I

Plants developed a diverse battery of defense mechanisms in response to continual challenges by a broad spectrum of pathogenic microorganisms. Their defense arsenal includes inhibitors of cell wall-degrading enzymes, which hinder a possible invasion and colonization by antagonists. The structure of Triticum aestivum xylanase inhibitor-I (TAXI-I), a first member of potent TAXI-type inhibitors of fungal and bacterial family 11 xylanases, has been determined to 1.7-A resolution. Surprisingly, TAXI-I displays structural homology with the pepsin-like family of aspartic proteases but is proteolytically nonfunctional, because one or more residues of the essential catalytical triad are absent. The structure of the TAXI-I.Aspergillus niger xylanase I complex, at a resolution of 1.8 A, illustrates the ability of tight binding and inhibition with subnanomolar affinity and indicates the importance of the C-terminal end for the differences in xylanase specificity among different TAXI-type inhibitors.status: publishe

Purification, crystallization and preliminary X-ray diffraction analysis of human phosphoserine phosphatase.

Phosphoserine phosphatase (PSP), a human enzyme involved in the L-serine biosynthesis pathway, has been crystallized using the hanging-drop vapour-diffusion method at 277 K. The crystals are orthorhombic, belonging to space group C222(1), with unit-cell parameters a = 49.03 A, b = 130.25 A, c = 157.29 A. Calculation of the Matthews coefficient indicates that there are two molecules in the asymmetric unit. A complete native data set to a resolution of 1.53 A has been collected at 100 K using synchrotron radiation

Crystallization and preliminary X-ray diffraction study of two complexes of a TAXI-type xylanase inhibitor with glycoside hydrolase family 11 xylanases from Aspergillus niger and Bacillus subtilis

Endo-beta-1,4-xylanases hydrolyze arabinoxylan, a major constituent of cereal cell walls, and are nowadays widely used in biotechnological processes. Purified complexes of family 11 xylanases from Aspergillus niger and Bacillus subtilis with TAXI I, a TAXI-type xylanase inhibitor from Triticum aestivum L., were prepared. In both cases the complex was crystallized using the hanging-drop vapour-diffusion method. The needle-like crystals of TAXI I in complex with A. niger xylanase belong to the trigonal space group P3(1) or P3(2), with unit-cell parameters a = b = 88.43, c = 128.99 A, and diffract to 1.8 A resolution. TAXI I in complex with B. subtilis xylanase crystallizes in the monoclinic space group C2, with a = 107.89, b = 95.33, c = 66.31 A, beta = 122.24 degrees. Complete data sets were collected for both crystal types using synchrotron radiation.journal: Acta Crystallographica Section D: Biological Crystallography content_type: crystallization papers peer_reviewed: Yes review_process: Single blind received: 27 October 2003 accepted: 18 December 2003 published_online: 25 February 2004 copyright: © 2004 International Union of Crystallographystatus: publishe

High-resolution structure of human phosphoserine phosphatase in open conformation.

The crystal structure of human phosphoserine phosphatase (HPSP) in the open conformation has been determined at a resolution of 1.53 A. The crystals are orthorhombic, belonging to space group C222(1), with unit-cell parameters a = 49.03, b = 130.25, c = 157.29 A. The asymmetric unit contains two molecules. Phase information was derived from a multiwavelength anomalous dispersion (MAD) experiment conducted at three wavelengths using a selenomethionine-derivative crystal of HPSP. The structure was refined using CNS to a final crystallographic R value of 21.6% (R(free) = 23.4%). HPSP is a dimeric enzyme responsible for the third and final step of the l-serine biosynthesis pathway. It catalyses the Mg2+-dependent hydrolysis of l-phosphoserine. Recently, the structure of HPSP in complex with an inhibitor bound to the active site has been reported to be the open conformation of the enzyme. Here, the structure of HPSP is reported in the absence of substrate in the active site. Evidence is presented that HPSP in an uncomplexed form is in an even more open conformation than in the inhibitor complex. In this state, the enzyme is partially unfolded to allow the substrate to enter the active site. Binding of the substrate causes HPSP to shift to the closed conformation by stabilizing the partially unfolded region. In the present structure a Ca2+ ion is bound to the active site and an explanation is given why HPSP is not active when in the active site Mg2+ is replaced by a Ca2+ ion