The conformation of serum albumin in solution: A combined phosphorescence depolarization-hydrodynamic modeling study

There is a striking disparity between the heart-shaped structure of human s erum albumin (HSA) observed in single crystals and the elongated ellipsoid model used for decades to interpret the protein solution hydrodynamics at n eutral pH. These two contrasting views could be reconciled if the protein w ere flexible enough to change its conformation in solution from that found in the crystal. To investigate this possibility we recorded the rotational motions in real time of an erythrosin-bovine serum albumin complex (Er-BSA) over an extended time range, using phosphorescence depolarization techniqu es. These measurements are consistent with the absence of independent motio ns of large protein segments in solution, in the time range from nanosecond s to fractions of milliseconds, and give a single rotational correlation ti me phi (BSA, 1 cP, 20 degreesC) = 40 +/- 2 ns. In addition, we report a det ailed analysis of the protein hydrodynamics based on two bead-modeling meth ods. In the first, BSA was modeled as a triangular prismatic shell with opt imized dimensions of 84 x 84 x 84 x 31.5 Angstrom, whereas in the second, t he atomic-level structure of HSA obtained from crystallographic data was us ed to build a much more refined rough-shell model. In both cases, the predi cted and experimental rotational diffusion rate and other hydrodynamic para meters were in good agreement. Therefore, the overall conformation in neutr al solution of BSA, as of HSA, should be rigid, in the sense indicated abov e, and very similar to the heart-shaped structure observed in HSA crystals.