1Beta sheets come in two flavors: parallel (shown on this slide) and anti parallel.The geometry of the individual beta strand is are almost identical in these twoforms of beta sheet. The difference is in the relative direction of neighboringstrands and in the way they hydrogen bond. Either way, just as an alpha helix, abeta sheet satisfies all hydrogen bonds of a peptide backbone. The onlyhydrogen bonds left un-bonded are those at the edges of the sheet.2Here is an antiparallel beta-sheet. Note how the overall appearance is verysimilar to the parallel beta sheet, even though the orientation of the strand and thedetailed hydrogen bonding pattern is different.Note the position of the side chains. They stick straight out of the plane of thebeta-sheet. The side chains are also sitting right next to one another. This sort ofarrangement allows the accommodation of large and beta-branched side chains.The interaction between these cross-strand neighboring side chains are a keyfactor in stabilizing beta sheets, or more precisely said reduces the energeticpenalty of forming a beta-sheet.3Neighboring beta-strands are often connected by turns. Turns are distinct fromloops, which involve several residues.By contrast, a turn is achieved by a single residue in which the peptide-bond isflipped by 90 degrees relative to the peptide bonds in the sheet. Another 90degree flip then positions the next peptide to be perfectly oriented to hydrogenbond back to the previous beta-strand.4Just like alpha helices beta-sheets are not particularly stable in isolation. Forexample, if one were to take a peptide with a sequence that is known to form abeta sheet in the context of a folded protein, chances are that this peptide will notform a stable beta-hairpin (I.e. two strands connected by a turn). It is onlythrough additional side chain - side chain interactions between multiple strands,that beta-sheet become stabilized.5Here is an example of how a beta-sheet can curl up onto itself to form astabilizing cross-sheet interactions that stabilize the beta-sheet conformation.6Here is another example in which a mostly betasheet structure can fold up toachieve stabilizing cross-strandinteractions7Beta sheets can be flat or have right-handed twistBetasheets are surprisingly twistable. While a beta sheet can be completely flat(I.e. the two strands lie on a flat plane) the two strands can also twist around oneanother quite dramatically. The beta-sheet on the right (shown in stereo)completes a full turn after about 12 aa acids. The twist parameter is defined asthe degrees by which a two peptide unit rotates relative to the previous 2 peptideunit. In other words the twist shown on the right corresponds to a 60 degreetwist. (60 x 12 / 2 = 360).8A survey of betasheet twists from the literature. As you can see there is acontinuous distribution of twists fromessentially 0 zero degree twist (completely flat) to a corkscrew like 72 degreetwist. However the twist, if present, isAlways right-handed. There are a number of factors that contribute to this righthandedness, in particular steric clashesBetween sidchains and the backbone. While no single interaction drives thehandedness of the twist, the origin of the twist and the absence of left-handedtwists can be traced back to basic physico-chemical interactions.9Individual beta strands in a beta sheet are often connected via short alpha helicesto form a beta-alpha-beta motif.10One feature of this beta-alpha-beta motif is that it occurs only in one of the twopossible topologies. Unlike the absence of left-handed twists of betasheets, thispreference for one of the conformations can not be traced back to unfavorableinteractions resulting from one of the two topologies. It therefore appears thatthis preference for one of the twoForms may have its origin in protein evolution.11This is the structure of Triose-phosphate isomerase (TIM) adopting the well-known TIM barrel fold.This is a particularly pretty way of combining beta-alpha-beta motifs to form aprotein