INVOLVEMENT OF LYSINE AND TRYPTOPHAN SIDE-CHAINS IN THE BIOTIN-AVIDININTERACTION

The biotin-1,4-diaminobutane (Bio-Put), biotin-tryptophan (Bio-Trp) an d biotin-3-methylindole (Bio-Sct) systems were investigated by means o f Raman spectroscopy. The interaction with Put renders the polar reson ance forms of Bio more stabilized. One NH2 group of Put is protonated by the COOH group of Bio and an electrostatic interaction between the negatively charged carboxylate group and the protonated amino group ta kes place. As far as the Bio-Trp and Bio-Sct systems are concerned, th e Raman spectra indicate that Trp and Set are not able to form stable complexes with Bio, and specific interactions between Bio and the indo lic ring do not occur. The Raman results indicate that the participati on of the ureido group of Bio in an extended hydrogen bond network gre atly contributes to the stabilization of the complex with proteins. Th ese specific interactions favour the dipolar resonance forms of Bio wh ich give rise to the formation of strong H-bonds with the protein. The Bio molecule is arranged in the active site in such a way that its ca rboxylic group orients towards a Lys residue, undergoing an ion-pair i nteraction with the amino group of Lys; however, this interaction appe ars to play a lesser role than that with the ureido ring. In spite of the many experimental results that have indicated the direct involveme nt of Trp in the formation of the Bio-avidin complex, none of the Trp residues binds the Bio molecule. The indolic rings probably interact w ith Bio via hydrophobic or charge-transfer forces, and are involved in the construction of the hydrophobic box in which Bio resides. (C) 199 7 by John Wiley & Sons, Ltd.