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Reversible membrane binding of gamma-carboxyglutamic acid (Gla)-containing coagulation factors requires Ca(2+)-binding to 10-12 Gla residues. Here we describe the solution structure of the Ca(2+)-free Gla-EGF domain pair of factor x which reveals a striking difference between the Ca(2+)-free and Ca(2+)-loaded forms. In the Ca(2+)-free form Gla residues are exposed to solvent and Phe 4, Leu 5 and Val 8 form a hydrophobic cluster in the interior of the domain. In the Ca(2+)-loaded form Gla residues ligate Ca2+ in the core of the domain pushing the side-chains of the three hydrophobic residues into the solvent. We propose that the Ca(2+)-induced exposure of hydrophobic side chains is crucial for membrane binding of Gla-containing coagulation proteins.

Cellobiohydrolase I (CBHI) is the major cellulase of Trichoderma reesei. The enzyme contains a discrete cellulose-binding domain (CBD), which increases its binding and activity on crystalline cellulose. We studied cellulase-cellulose interactions using site-directed mutagenesis on the basis of the three-dimensional structure of the CBD of CBHI. Three mutant proteins which have earlier been produced in Saccharomyces cerevisiae were expressed in the native host organism. The data presented here support the hypothesis that a conserved tyrosine (Y492) located on the flat and more hydrophilic surface of the CBD is essential for the functionality. The data also suggest that the more hydrophobic surface is not directly involved in the CBD function. The pH dependence of the adsorption revealed that electrostatic repulsion between the bound proteins may also control the adsorption. The binding of CBHI to cellulose was significantly affected by high ionic strength suggesting that the interaction with cellulose includes a hydrophobic effect. High ionic strength increased the activity of the isolated core and of mutant proteins on crystalline cellulose, indicating that once productively bound, the enzymes are capable of solubilizing cellulose even with a mutagenized or with no CBD.

Molecular dynamics simulations of a sodium octanoate micelle in aqueous solution are reported. Simple analytical expressions have been used for the interatomic interactions, including a Lennard-Jones term and a Coulombic interaction betwen partial charges on each site. Two different interaction potentials have been investigated, and amphiphilic aggregation is shown to depend strongly on the electrostatic properties of the water model. For both potentials the micelle shows a broad transition region between the aqueous and hydrocarbon regions. The hydration of different carbon atoms in the chain is largest for the carboxylic atom, then decreases along the chain, reaching a minimum and then increases again at the end of the chain. The micellar translational and rotational diffusion are too fast, probably due to deficiencies in the water model. The rotational diffusion of the entire micelle is found to be an order of magnitude slower than that of the monomers. Reorientational time correlation functions and order parameters for C–H vectors in the chains are reported and agree qualitatively with a recently proposed two-step model for NMR relaxation in micelles. The dynamics of water molecules close to the micellar surface is found to be similar to bulk water.

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