The Rossmann fold is commonly identified as a super-secondary structure in protein folding. The fold is composed of numerous alternating beta strand (b) and alpha helical (a) segments wherein the b-strands are hydrogen bonded forming a beta-sheet. The initial beta-alpha-beta (bab) fold is the most conserved segment of Rossmann folds. The motif is named after Michael Rossman who first discovered this fold in the enzyme lactate dehydrogenase in 1970 and pointed out that this was a frequently occurring motif in nucleotide binding proteins (NAD-binding proteins).
The Rossmann fold, or rather the Rao-Rossmann fold, is a protein structural motif found in proteins that bind nucleotides, such as enzyme cofactors FAD, NAD+, and NADP+. Although it was never defined, the term fold was coined in 1973 by Rao and Rossmann in relation to nucleotide-binding proteins.The Rossmann fold was explained by Dr. Michael Rossmann and coworkers in 1974. He was the first to deduce the structure of lactate dehydrogenase and found the structural motif of Rossmann folding. It was found that most dehydrogenases that utilize NAD or NADP bind the cofactor in a conserved protein domain known as the Rossmann fold. They described that this folding was responsible for the binding of dinucleotide coenzymes in different proteins. The motif is named for Michael Rossmann, who first pointed out that this is a frequently occurring motif in nucleotide binding proteins, such as dehydrogenases. In 1989, Israel Hanukoglu from the Weizmann Institute of Science discovered that the consensus sequence for NADP+ binding site in some enzymes that utilize NADP+ differs from the NAD+ binding motif. This discovery was used to re-engineer coenzyme specificities of enzymes.
The Rossmann fold is a structral motif evident in the NAD-binding site of numerous dehydrogenases. The structure is composed of up to six mostly parallel beta strands. The first two strands are connected by an α- helix and the motif is deduced to be beta-alpha-beta-alpha-beta. Certain Rossmann fold proteins include only one copy of the beta-alpha-beta-alpha-beta structure, with additional beta-strands that form hydrogen-bonds with the previous one. One of the other differences in Rossmann folds in proteins has to do with enzyme-specificity toward the co-enzyme. Through the analysis of four NADH-binding enzymes, it was found that in all four enzymes the nucleotide co-enzyme entailed the same conformation and orientation with respect to the polypeptide chain. After the finding of Rossmann fold, it has been identified in numerous enzymes that utilize the dinucleotide co-enzymes NADH, NADPH and FADH2. Another type of fold is determined to be a “P-loop” fold. The evolutionary relationship between the Rossmann fold and Rossmann-like folds is unclear. These folds are referred to as Rossmannoids. It is believed by scientists that all these folds, including a Rossmann fold originated from a single common ancestral fold, that had nucleotide binding capabilities, in addition to non-specific catalytic activity. 
The structures of Rossmann fold segments between additional strands vary greatly and may be composed of a variety of structures such as multiple short helices or coils. The Rossmann fold is a commonly distributed super-secondary structure. It entails a series of alternating beta strand and alpha helical segments. The most conserved segment of Rossmann folds is the primary beta-alpha-beta fold. Since this segment is in contact with the ADP portion of dinucleotides such as FAD, NAD and NADP it is also called as an "ADP-binding beta-beta fold. Certain Rossmann fold proteins include only one copy of the beta-alpha-beta-alpha-beta structure, with additional beta-strands that form hydrogen-bonds with the previous one. One of the other differences in Rossmann folds in proteins has to do with enzyme-specificity toward the co-enzyme.
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