Hydrogen bonds are identified by arrows connecting the donor nitrogen and acceptor oxygens. The three antiparallel strands are shown in both cartoon format (left) and in stick form containing backbone atoms N, CA, C, and O' (right). The figure to the right shows a three-stranded antiparallel beta sheet from thioredoxin. The distance between successive hydrogen bonds alternates between shorter and longer. Strands are numbered according to their relative position in the polypeptide sequence.Ģ) Antiparallel beta sheet - The beta strands run in alternating directions and therefore can be quite close on the primary sequence. The three parallel strands are shown in both cartoon format (left) and in stick form containing backbone atoms N, CA, C, and O' (right). The figure to the left shows a three-stranded parallel beta sheet from the protein thioredoxin. The hydrogen bonds are equally distanced. As a result they have to be separated by long sequence stretches. Types of Beta Sheets Observed in Proteinsġ) Parallel beta sheet - All bonded strands have the same N to C direction. Note that most sequential pairs are next to each other, and that the chain starts in the middle, moves to one edge, skips back to the middle and then moves out to the other edge.There are three possible ways to form a beta sheet from beta strands, discussed below. The strand labels show strand sequence order. Click on atoms along a strand to tell its direction from the residue numbers, and satisfy yourself that all six strands are indeed parallel. This twist is usually described by the twist in orientation of the peptide planes (or H-bond plane) as one progresses along the strand by this definition beta sheet twist is always right-handed, although by varying amounts. Drag right or left to better see that the sheet as a whole twists. Notice that the H-bonds in this parallel shet are slanted in alternate directions, rather than perpendicular to the strands as we will see in antiparallel sheets. Lactate dehydrogenase is the classic, first-seen example of this type of structure and has the most frequently-observed topology of beta connections. This "fold" is also known as the "nucleotide-binding domain", because most examples bind a mononucleotide (such as FMN) or a dinucleotide (such as NAD) near the middle of one end of the beta sheet. This doubly-wound parallel beta sheet is the most common folding pattern found in known protein structures. Kinemage 1 shows the 6-stranded parallel beta sheet from domain 1 of lactate dehydrogenase (file 1LDM). To view a beta sheet in the KiNG Java Applet, click here. Approximately a quarter of all residues in a typical protein are in beta strands, though this varies greatly between proteins the strands forming a beta sheet can be separated in primary structure by long sequences of amino acids that are not part of the sheet. The participating beta strands are not continuous in the primary sequence, and do not even have to be close to each other in the sequence, i.e. Adjacent beta strands can hydrogen bond to form a beta sheet (also referred to as a beta pleated sheet). A beta strand is an element of secondary structure in which the protein chain is nearly linear.
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