Protein A from Staphylococcus aureus

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Abstract
Protein A, a cell-wall protein from Staphylococcus aureus, has been studied by spectrophotometry and spectropolarimetry. All the four tyrosines are similarily titrated with pKa= 10.25. Circular dichroism (CD) spectra show that the conformation of protein A is very stable over a large pH interval (0.99–11.8). The conformation is partly intact even in 6 M guanidine hydrochloride and at 80°C. Protein A contains about 50%α-helical structure and 10–20%β-structures. CD band maxima at 261 and 268 nm are ascribed to transitions in the phenylalanine residues and ellipticities between 275–285 nm to the tyrosine residues. Of the four tyrosines, 3.5 are perturbed by 20% polyethylene glycol, while all of them are perturbed by 20% dimethylsulfoxide. The role of the tyrosines in the reaction with the Fc-region of immunoglobulins is discussed.