6.0 Mixed Beta strand and Helix Supersecondary Structures

These supersecondary structures are a combination of both beta strands and helices.

6.1 Beta-Alpha-Beta

• Figure 6.0 shows that the alpha helix allows the connection between two adjacent parallel beta strands. The helix joins the C-terminal of one strand (through a connecting loop) and the N-terminal of the adjacent strand (again through a connecting loop). Consequently, and as the name suggests, the secondary structures are arranged sequentially, from N-terminal to C-terminal, with a beta strand followed by an alpha helix followed by a beta strand.

• The helix joins parallel beta strands and is itself almost parallel with the two beta strands

• There are two orientations to this structure: right handed and left handed. Figure 6.1 shows that if you viewed the motif from the N-terminal ends of the beta strands , and one strand was above the other in the same plane, then the right hand orientation is when the helix is on the 'right-hand' side of the plane. A left handed orientation would have the helix on the left hand side of the plane.

• The right-hand orientation of the structure is the most abundant in proteins

• This structure exists in almost all proteins with parallel strands

• The loops connecting the strands to the helix are variable length and often involved in ligand binding.

6.2 Rossmann Fold

• Figure 6.2 shows that the Rossmann fold is a combination of three beta strands and two helices arranged sequentially such that the beta strands and the helices alternate within the structure. That is, the sequence of secondary structures from N-terminal to C-terminal is: beta strand, helix, beta strand, helix, beta strand.

• Present in many proteins which bind nucleotides (eg NAD) such as lactate dehydrogenase.

• Could be seen as two beta-alpha-beta motifs joined together

• Important function by binding NAD within most dehydrogenases.

• Rossmann folds tend to reverse the direction of the chain within a protein

• Named after Michael Rossmann who first suggested that this was a common motif in many nucleotide binding proteins.