Aqueous-organic solutions of enzymes at sub-zero temperatures

Density and electron density of aqueous cryoprotectant solutions at cryogenic temperatures for optimized cryoprotection and diffraction contrast

The glass-phase densities at T = 77 K of aqueous solutions of the common cryoprotective agents (CPAs) methanol, ethanol, 2-propanol, glycerol, 2-methyl-2,4-pentanediol (MPD), ethylene glycol, polyethylene glycol 200 and polypropylene glycol 425 were measured as a function of CPA concentration. Individual drops with volumes as small as ∼65 pl were rapidly cooled to achieve the glass phase, and their densities at T = 77 K were determined by cryoflotation. These densities were used to determine the glass-phase electron density of each solution and its volume thermal contraction between room temperature and 77 K. When combined with data for the critical cooling rates required to achieve the glass phase versus CPA concentration, these yield alternative measures of cryoprotectant effectiveness. These reference data will aid in minimizing sample stresses and mechanical damage in cryocrystallography, in cryogenic temperature X-ray imaging and in vitrification-based cryopreservation protocols, and in maximizing electron-density contrast between cryoprotectant solutions and biomolecules in cryogenic temperature small-angle X-ray scattering experiments and cryo-electron microscopy.

Nonconventional protease catalysis in frozen aqueous solutions

During the past decade proteases have been widely used as catalysts in peptide synthesis. Unfortunately, they are not ideal ligases. Enzymatic peptide synthesis in frozen aqueous systems has been developed as an approach towards the suppression of competitive reactions. This paper summarizes reports concerning the behaviour of non-enzymatic as well as of enzyme-catalysed reactions when the reaction mixture is frozen. The advantages of freezing the reaction mixture in serine and cysteine protease-catalysed peptide synthesis, the influence of modified reaction conditions and the possible reasons for the yield-increasing effect of freezing are discussed.

Developments in low-temperature biochemistry and biology

Though under most circumstances harmful changes are induced in cellular structures by subzero temperatures, conditions can be found under which such damage is avoided. Thus, in solution, biochemical reactions can be slowed and more easily analysed and many enzyme-substrate complexes can be stabilized and structurally analysed; in crystals, 'stop-action' pictures unveil the stereochemical changes along reaction pathways. The progressive 'solidification' of non-covalent bonds involved in protein structures should permit investigation of their dynamics. Studies at high pressures open the way to new investigations on 'activated' enzyme-substrate complexes and might permit the refinement of current concepts to a considerable degree, as a preliminary but decisive step towards a full description of enzyme mechanisms. The conditions of medium allowing such cryobiochemical studies fail to protect cellular structures against cold. Investigations of plasma membrane behaviour are now under way to determine processes leading to cryosensitivity or cryotolerance of cells.

Formation of stable crystalline enzyme-substrate intermediates at sub-zero temperatures

Sub-zero temperatures can be used to trap intermediates in enzyme-catalysed reactions using suitable cryosolvents. The feasibility of obtaining such intermediates in the crystalline state for X-ray diffraction studies has been demonstrated with several proteases, using specific substrates and optimal pH.

Crystal structure of elastase-substrate complex at -- 55 degrees C

The structure of a specific acyl-enzyme intermediate in the elastase-catalysed hydrolysis of N-carbobenzoxy-L-alanyl-p-nitrophenol ester has been determined by X-ray diffraction at 3.5 A resolution. The acyl-enzyme was stabilised by cooling the crystal to --55 degrees C during substrate addition and data collection.