This protein contains both alpha helices and beta sheets. There are two main classes of these proteins. One is the beta-barrel of TIM. This class consists of open twisted beta-sheets structure with helices on both sides of the sheet. These structures can differ greatly (in the number of beta strands (4-10) and in the fact that beta strands continuous in the primary structure may not be adjacent in the beta sheet). The active site is a crevice comprised of the region around the carboxy-terminus of the parallel beta strands.