title: Critical importance of loop conformation to avidin-enhanced hydrolysis of an active biotin ester.
abstract: The homotetrameric and biotin-binding properties of avidin and streptavidin have been exploited for a myriad of biotechnological applications and theoretical studies. Among the few differences between the two proteins is the capacity of avidin to hydrolyze biotinyl p-nitrophenyl ester (BNP), as opposed to streptavidin, which fully protects the same pseudosubstrate from hydrolysis. Combined mutagenesis and X-ray analysis have been used to attempt to understand this diametric difference in activities. It was found that a charged residue and one of the loops (L3,4) are together responsible for this difference. Recently, the avidin-related analogue AVR4 was found to have an even more pronounced BNP-hydrolysis activity than avidin. Again, the combination of charged residue(s) (Asp39 and/or Arg112) and the rigid conformation of the L3,4 loop was suggested to be responsible for the observed hydrolysis reaction. However, replacement of the latter charged residues in AVR4 resulted in only a modest reduction in hydrolytic activity at most, whereas replacement of the L3,4 loop of avidin with the rigid loop of AVR4 caused a dramatic increase in the activity of avidin. These results clearly demonstrate that the main feature responsible for the observed differences in rates of hydrolysis among the avidins is the conformational status of the L3,4 loop, which imposes conformational constraints on the pseudosubstrate, thereby rendering it susceptible to nucleophilic attack by solvent. In this context, the hydrolytic properties of the avidins reflect enzyme catalysis, in that subtleties in substrate binding are the determining features of catalytic efficiency.
authors.affiliation: Department of Biological Chemistry, The Institute of Life Sciences, The Wolfson Centre for Applied Structural Biology, The Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel.
authors.forename: Ruchama
authors.initials: R
authors.lastname: Hayouka
authors.forename: Yael
authors.initials: Y
authors.lastname: Eisenberg-Domovich
authors.forename: Vesa P
authors.initials: VP
authors.lastname: Hytönen
authors.forename: Juha A E
authors.initials: JA
authors.lastname: Määttä
authors.forename: Henri R
authors.initials: HR
authors.lastname: Nordlund
authors.forename: Markku S
authors.initials: MS
authors.lastname: Kulomaa
authors.forename: Meir
authors.initials: M
authors.lastname: Wilchek
authors.forename: Edward A
authors.initials: EA
authors.lastname: Bayer
authors.forename: Oded
authors.initials: O
authors.lastname: Livnah
mesh_terms: D001360:Avidin / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D001710:Biotin / Q000378:metabolism*; D006868:Hydrolysis / Q000187:drug effects; D011487:Protein Conformation; D017434:Protein Structure, Tertiary
chemical_list: D001360:Avidin; D001710:Biotin