posted on 2021-08-23, 12:34authored byRalph Maier, Madeleine R. Fries, Cara Buchholz, Fajun Zhang, Frank Schreiber
The protein human
serum albumin (HSA) is able to readily crystallize
in the presence of trivalent cations, whereas this is not the case
for the homologous protein in cattle, bovine serum albumin (BSA),
although both have analogous functions as well as similar physicochemical
properties. To understand the underlying interactions and mechanisms,
we investigated their bulk phase behavior with CeCl3 by
visual inspection, optical microscopy, and small-angle X-ray scattering
(SAXS). The results reveal that both proteins undergo reentrant condensation
and liquid–liquid phase separation (LLPS). However, the LLPS
binodal for HSA shifts toward lower protein concentrations than that
for BSA, indicating a stronger intermolecular attraction in HSA solutions
at the same compositions, consistent with SAXS measurements. Moreover,
crystallization occurs within the condensed regime of HSA, but no
crystallization was observed for BSA. Adsorption studies at a hydrophilic
SiO2 surface demonstrate that both systems show reentrant
adsorption with a higher amount of adsorbed BSA, likely due to enhanced
cation-mediated interactions and/or hydrogen bonds. We conclude that
the higher surface hydrophobicity of HSA could explain the experimental
observations. These additional hydrophobic interactions not only strengthen
the attraction between the proteins but also provide directional and
specific protein–protein contacts, which are favored for protein
crystallization. This work further demonstrates the sensitivity and
complexity of protein interactions in solution: subtle differences
in molecular structure lead to a dramatic change in their phase behavior.
Generalization of these findings can pave the way toward, e.g., better
drug design and improve medical treatment.