Kalevi Visuri

Novel reactions of xylose isomerase from Streptomyces rubiginosus

Abstract Three novel substrates— d -arabinose, l -ribose, and d -lyxose—are described for the industrially important xylose isomerase (glucose isomerase). Furthermore, it was shown that contrary to previous studies, two different reaction products are formed when pentose sugars are used as substrates. In extended reactions an apparent equilibrium is formed between both d - and l -forms of arabinose, ribulose, and ribose as well as d -xylose, d -lyxose, and d -xylulose. The new reactions are discussed in the light of previously described reaction mechanisms for this enzyme.

Stability of native and cross-linked crystalline glucose isomerase.

Stabilities of native and cross-linked crystalline forms of Streptomyces rubiginosus glucose isomerase were compared in buffer and in 45% glucose/fructose solutions. The cross-linked crystalline form of the enzyme was more stable in the presence of substrate while in a buffer solution the native enzyme was more stable. Inactivation of native enzyme in buffer did not obey first-order kinetics but proceeded with a rapid first phase followed by a stable phase. This stabilization is interpreted to be a result of a conformational change in the protein structure. Inactivation of the native enzyme in buffer was directly related to protein precipitation. In the presence of high substrate concentration, the inactivation was related to browning reactions between the enzyme and the reactive sugar, resulting in soluble sugar-protein complexes.

Xylitol purification by cross-linked glucose isomerase crystals

Xylitol is specifically bound by active, cross-linked glucose isomerase crystals (CLGI). CLGI can be used to purify xylitol or concentrate it from dilute and impure solutions. Bound xylitol can be eluted from CLGI by Ca2 and the material reactivated by Mg2. The binding capacity is 1 mg xylitol per 525 mg CLGI which equals one molecule per active center. CLGI can further be used to purify xylitol and sorbitol from impure mixtures of arabinitol, mannitol, ribitol and monosaccharides.

Purification, crystallization and preliminary X-ray diffraction analysis of the Trichoderma reesei hydrophobin HFBI.

Hydrophobins are fungal proteins that are capable of altering the hydrophobicity of surfaces by self-assembly at hydrophilic-hydrophobic interfaces. Here, the growth of hydrophobin crystals suitable for X-ray crystallography is reported. The hydrophobin HFBI from Trichoderma reesei was crystallized by vapour diffusion in hanging drops in 30% PEG 4000, 0.1 M sodium citrate pH 4.3 buffer containing 0.2 M ammonium acetate and CYMAL-5 detergent (initial concentration of 2.4 mM). HFBI crystals are hexagonal and belong to space group P6(1) (or P6(5)), with unit-cell parameters a = b = 45.9, c = 307.2 A. The HFBI used in the crystallization experiments was purified from fungal cell walls.