Adherence of Packing Defects in Soluble Proteins

For protein structure to prevail in water, its backbone hydrogen bonds must be shielded from water attack, requiring a cluster of “wrapping” nonpolar groups. Thus, underwrapped regions are adhesive, as exogenous removal of surrounding water becomes thermodynamically favorable. Here we measure the average adhesive force exerted by an underwrapped hydrogen bond on a test hydrophobe and thus define a new interactivity constant.

Figure 1

Pattern of UWHB’s in the crystal structure of human apomyoglobin (a), lysozyme (b), and ${\beta }_2$ microglobulin (c). The ribbon representations are an aid to the eye. The backbone is represented as a blue virtual-bond polygonal joining $\alpha$ carbons, the well wrapped backbone hydrogen bonds are shown as grey segments joining the $\alpha$ carbons of the residues involved, and the UWHBs ($\rho <12$) are displayed as green segments.

Figure 2

Representation of the adsorption experiment. The soluble protein molecule enters the cell and faces one of three possible outcomes as a result of its Brownian motion: (I) it does not land in time before exiting; (II) it lands on the bilayer and the landing is successful because the bilayer faces the adhesive UWHB; and (III) it lands in time but the landing is not sticky since the bilayer faces a well-wrapped side of the protein. The UWHB is represented as a thick segment.

Figure 3

Adsorption uptake at 900 s for $\beta$ lactoglobulin (filled diamonds), apomyoglobin (open circles), lysozyme (open squares), ${\beta }_2$ microglobulin (filled triangles), and apolipoprotein A-I and monomeric human insulin (open triangles) onto the DLPC bilayer. The values for the apolipoprotein and insulin are practically indistinguishable, with the expected values for one protein lying within the dispersion of the other, and vice versa. The uptake is given in thousands of molecules per $\mu {\mathrm{m}}^2$ and plotted as a function of the flow rate $\omega$. The dispersion in the ordinate values results from 15 repetitions of the adsorption experiment under the same hydrodynamic conditions and bulk composition. The solid lines represent the theoretical computation fixing $\Delta U$ at $3.91\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$. The thicker solid lines correspond to $\beta$ lactoglobulin, lysozyme, and apolipoprotein A-I.