Crambe abyssinica (Abyssinian cabbage)
Analysis of the genes encoding protein shows that on average proteins are about 350 residues long. Compared to such average proteins, crambin is a rather small protein with 46 amino acid residues, only about one-tenth of the size of a regular protein. Crambin is found in the plant seeds of Abyssinian cabbage. Its biological function is unknown and it is not related to any human diseases. As such it seems a rather uninteresting and atypical molecule.
It has two alpha-helices and two beta-strands forming an anti-parallel sheet. It has six cysteine residues, accounting for 13% of the structure, an unusually high number. It has been studied extensively both theoretically and experimentally, because crystals of crambin diffract extremely well. The structure of crambin has been determined at a resolution of 0.54 angstrom, the best ever reported for a protein. The structure of crambin is however notable for historical reasons. When the crystallization of crambin was reported in 1979 by Hendrickson and Teeter, it was remarkable for its excellent diffraction, to less than 0.8 A resolution. While there are now more protein crystals diffracting to such high resolution, it was, at that time, a very unusual result. The only available experimental method to determine unknown structures then was multiple isomorphous replacement (MIR), developed by Perutz for his work on hemoglobin. Hendrickson tried to use MIR, but failed. In an act of desperation he tried to use the so-called anomalous diffraction of the sulfur atoms of cysteine to determine the structure of crambin with conventional x-ray radiation. It was considered impossible, because the anomalous part of diffraction is weak, but he succeeded. This was a breakthrough result in protein crystallography. The method is now known as single-wavelength anomalous diffraction (SAD) and used quite often. Building on these results, Hendrickson and coworkers then developed the method of multi-wavelength anomalous diffraction (MAD) and in 1987, for the first time, Hendrickson, Paehler and Smith succeeded to determine an unknown protein structure, a complex of streptavidin with selenobiotin, by this method. Because MAD is easier to use and more accurate than MIR, it is nowadays the method of choice for the determination of new protein structures, being used in more than 40% of all new structures deposited to the PDB. It is beginning to replace MIR, which Max Perutz had pioneered for the structure determination of hemoglobin. Crambin is the molecule which started this development.
Protein Data Bank (PDB)
Teeter, M.M.; "Water Structure of a Hydrophobic Protein at Atomic Resolution. Pentagon Rings of Water Molecules in Crystals of Crambin"; Proc.Natl.Acad.Sci.USA; (1984) 81:6014-6018. PubMed:16593516
author: Arno Paehler