Alpha-lactalbumin forms a compact molten globule in the absence of disulfide bonds.
Human alpha-lactalbumin (alpha-LA) is a four disulfide-bonded protein that adopts partially structured conformations under a variety of mildly denaturing conditions. At low pH, the protein is denatured but compact, with a high degree of secondary structure and a native-like fold. This is commonly referred to as a molten globule. A variant of alpha-LA, in which all eight cysteines have been mutated to alanine (all-Ala alpha-LA), has been studied using NMR spectroscopy. At low pH all-Ala alpha-LA is nearly as compact as wild type alpha-LA. Urea-induced unfolding experiments reveal that the residues that remain compact in the absence of disulfide bonds are those that are most resistant to unfolding in the wild-type alpha-LA molten globule. This is particularly remarkable because this stable core is formed by segments of the polypeptide chain from both the N- and C-termini. These results show that the overall architecture of the protein fold of alpha-LA is determined by the polypeptide sequence itself, and not as the result of cross-linking by disulfide bonds, and provide insight into the way in which the sequence codes for the fold.
Redfield
C
C
Oxford Centre for Molecular Sciences, New Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QT, UK. redfield@bioch.ox.ac.uk
Schulman
B A
BA
Milhollen
M A
MA
Kim
P S
PS
Dobson
C M
CM
D019943:Amino Acid Substitution; D002942:Circular Dichroism; D003545:Cysteine / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004058:Diffusion; D004220:Disulfides / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007768:Lactalbumin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D014508:Urea
D004220:Disulfides; D014508:Urea; D007768:Lactalbumin; D003545:Cysteine