PMID: 17447762

Authors:

Vasileiou C, Vaezeslami S, Crist RM, Rabago-Smith M, Geiger JH, Borhan B

Title:

Protein design: reengineering cellular retinoic acid binding protein II into a rhodopsin protein mimic.

Journal:

J Am Chem Soc. 2007 May 16;129(19):6140-8. Epub 2007 Apr 21.

Abstract:

Rational redesign of the binding pocket of Cellular Retinoic Acid Binding Protein II (CRABPII) has provided a mutant that can bind retinal as a protonated Schiff base, mimicking the binding observed in rhodopsin. The reengineering was accomplished through a series of choreographed manipulations to ultimately orient the reactive species (the epsilon-amino group of Lys132 and the carbonyl of retinal) in the proper geometry for imine formation. The guiding principle was to achieve the appropriate Burgi-Dunitz trajectory for the reaction to ensue. Through crystallographic analysis of protein mutants incapable of forming the requisite Schiff base, a highly ordered water molecule was identified as a key culprit in orienting retinal in a nonconstructive manner. Removal of the ordered water, along with placing reinforcing mutations to favor the desired orientation of retinal, led to a triple mutant CRABPII protein capable of nanomolar binding of retinal as a protonated Schiff base. The high-resolution crystal structure of all-trans-retinal bound to the CRABPII triple mutant (1.2 A resolution) unequivocally illustrates the imine formed between retinal and the protein.