Is cysteine residue important in FITC-sensitive ATP-binding site of P-type ATPases? A commentary to the state of the art

Treatment of P-type ATPases (from mammalian sources) by fluorescein isothiocyanate (ITC) revealed the ITC label on a lysine residue that was than considered as essential for binding of ATP in the ATP-binding sire of these enzymes. On the other hand, experiments with site directed mutagenesis excluded the presence of an essential lysine residue that would be localized in the ATP binding sites of ATPases. Other previous studies, including those of ourselves, indicated that the primary site of isothiocyanate interaction may be the sulflhydryl group of a cysteine residue and this may be essential for binding of ATP. In addition considerable knowledge accumulated since yet also about the differences in stability of reaction product of isothiocyanates with SH- or NH2-groups. Based upon evaluation of the data available up to now, in present paper the following tentative roles for lysine and cysteine residues located in the ATP-binding site of P-type ATPases are proposed: The positively charged micro-domain of the lysine residue may probably attract the negatively charged phosphate moiety of the ATP molecule whereas the cysteine residue may probably be responsible for recognition and binding of ATP by creation of a proton bridge with the amino group in position 6 on the adenosine ring of ATP.