I. Zhukov et al., Conservative mutation Met8 -> Leu affects the folding process and structural stability of squash trypsin inhibitor CMTI-I, PROTEIN SCI, 9(2), 2000, pp. 273-279
Protein molecules can accommodate a large number of mutations without notic
eable effects on their stability and folding kinetics. On the other hand, s
ome mutations can have quite strong effects on protein conformational prope
rties. Such mutations either destabilize secondary structures, e.g., alpha-
helices, are incompatible with dose packing of protein hydrophobic cores, o
r lead to disruption of some specific interactions such as disulfide cross
links, salt bridges, hydrogen bonds, or aromatic-aromatic contacts. The Met
8 --> Leu mutation in CMTI-I results in significant destabilization of the
protein structure. This effect could hardly be expected since the mutation
is highly conservative, and the side chain of residue 8 is situated on the
protein surface. We show that the protein destabilization is caused by rear
rangement of a hydrophobic cluster formed by side chains of residues 8, Ile
6, and Leu17 that leads to partial breaking of a hydrogen bond formed by th
e amide group of Leu17 with water and to a reduction of a hydrophobic surfa
ce buried within the cluster. The mutation perturbs also the protein foldin
g, In aerobic conditions the reduced wild-type protein folds effectively in
to its native structure, whereas more then 75% of the mutant molecules are
trapped in various misfolded species. The main conclusion of this work is t
hat conservative mutations of hydrophobic residues can destabilize a protei
n structure even if these residues are situated on the protein surface and
partially accessible to water. Structural rearrangement of small hydrophobi
c clusters formed by such residues can lead to local changes in protein hyd
ration; and consequently, can affect considerably protein stability and fol
ding process.