Two of the commonest protein "folds" in 3D, each illustrated by a very simplified schematic diagram (at right) of the course of the polypeptide chain, and by a ribbon drawing of the classic example protein (at left). First (top) is the fold called a "TIM barrel", (beta/alpha)8 barrel, or "singly-wound" barrel, with a central cylinder of 8 parallel beta strands, adjacent strands connected by righthanded beta-alpha-beta loops. Triose phosphate isomerase (TIM) is the classic example. Second (below) is the fold called a nucleotide-binding domain, a "Rossmann fold", or a "doubly-wound" parallel beta domain, with a central, twisted sheet of parallel beta strands, The chain starts at bottom center, winds to the right with beta-alpha-beta loops (black half in ribbon drawing), then switches back to the center and winds to the left. Lactate dehydrogenase (LDH) domain 1 is the classic example