Enzyme ligand complexes: spectroscopic studies

Probing the conformation and orientation of adsorbed enzymes using side-chain modification

The bioactivity of enzymes that are adsorbed on surfaces can be substantially influenced by the orientation of the enzyme on the surface and adsorption-induced changes in the enzyme's structure. Circular dichroism (CD) is a powerful method for observing the secondary structure of proteins; however, it provides little information regarding the tertiary structure of a protein or its adsorbed orientation. In this study, we developed methods using side-chain-specific chemical modification of solvent-exposed tryptophan residues to complement CD spectroscopy and bioactivity assays to provide greater detail regarding whether changes in enzyme bioactivity following adsorption are due to adsorbed orientation and/or adsorption-induced changes in the overall structure. These methods were then applied to investigate how adsorption influences the bioactivity of hen egg white lysozyme (HEWL) and glucose oxidase (GOx) on alkanethiol self-assembled monolayers over a range of surface chemistries. The results from these studies indicate that surface chemistry significantly influences the bioactive state of each of these enzymes but in distinctly different ways. Changes in the bioactive state of HEWL are largely governed by its adsorbed orientation, while the bioactive state of adsorbed GOx is influenced by a combination of both adsorbed orientation and adsorption-induced changes in conformation.

Determination of the folding of proteins as a function of denaturants, osmolytes or ligands using circular dichroism

Circular dichroism (CD) is an excellent tool for examining the interactions and stability of proteins. This protocol covers methods to obtain and analyze circular dichroism spectra to measure changes in the folding of proteins as a function of denaturants, osmolytes or ligands. Applications include determination of the free energy of folding of a protein, the effects of mutations on protein stability and the estimation of binding constants for the interactions of proteins with other proteins, DNA or ligands, such as substrates or inhibitors. The experiments require 2-5 h.

Temperature dependence for fluorescence of beta-NADH in glycerol/water solution and in trehalose/sucrose glass

Fluorescence imaging of cells and tissue can be used to evaluate beta-NADH redox and location. At low temperature, beta-NADH fluorescence intensity increases and therefore sensitivity of imaging increases. In this paper, the temperature dependence of fluorescence was evaluated for beta-NADH in glycerol/water solution and in trehalose/sucrose glass. The average fluorescence lifetime for NADH in glycerol/water is 0.66 ns, compared with 5.3 ns in trehalose/ sucrose at 20 degrees C. Emission spectra were recorded from 290 to 12 K. The fluorescence of beta-NADH in glycerol/water increases approximately 16 fold and the emission shifts about 35 nm to the blue as temperature decreases. Much smaller change is seen for fluorescence of beta-NADH in sugar glass. Below 77 K, the beta-NADH spectral features did not change significantly with temperature change, and so no increase in sensitivity is obtained by going to very low temperatures. It is suggested that the sensitivity of beta-NADH fluorescence is related to water relaxation around the excited state molecule. Differences in water in various tissues may contribute to beta-NADH fluorescence changes when cells are altered.

Spectroscopic studies on pepsin-solubilized vitreous and cartilage collagens

Circular dichroism and the fluorescent probe, 2-p-toluidinyl-naphthalene-6-sulfonate, were used to compare the molecular properties of pepsin-solubilized vitreous collagen with cartilage and calfskin collagens. Type II vitreous and cartilage collagens have more hydrophobic regions along their molecular domain than does type I calfskin collagen. The rate of fibril growth is faster in type II collagens than in type I. The increased hydrophobicity of type II collagens is attributed to high carbohydrate content and compositional variations. Although the secondary structures of the three collagens do not differ significantly, differences in carbohydrate content, composition, and hydrophobic character may cause some variations in the tertiary structures. It is suggested that the tertiary structure plays an important role in the nature and rate of fibril growth. Differences between cartilage and vitreous collagen in fluorescence behavior, fibril growth, and melting temperature indicate that vitreous collagen may be a "special type II collagen."