Peptides seven-residue, crystal structure

A. Crystal Structure of a Seven-Residue Peptide from Sup35p. 162... 

Recently, a seven-residue peptide from the Sup35p prion domain has been analyzed by X-ray crystallography (Nelson et al., 2005). Although there is, in general, little reason to suppose that a short peptide will assume the same structure in a crystal as it will in the context of a folded protein containing it, these crystals seem to be related to amyloid fibrils of the same... 

Aspartic proteases form a group of proteolytic enzymes that catalyze peptide bond cleavage by acid-base catalysis and activation of a water molecule for nucleophilic attack on the amide carbon. Crystal structures of mammalian and fungal enzymes are known. In pepsin, the best studied aspartic protease, catalysis proceeds by water activation and leaving group protonation. Both involve an aspartate, which explains the low pH optimum of around 4. The two aspartic acid residues are situated around a hydrophobic deft that can accommodate seven amino acids (the S4-S3 subsites). The active site is covered by a flexible flap, which contributes to SI subsite specificity. 

Integral membrane proteins. Membrane proteins are hard to crystallize and precise structures are known for only a few of thern." " A large fraction of all of the integral membrane proteins contain one or more membrane-spanning helices with loops of peptide chain between them. Folded domains in the cytoplasm or on the external membrane surface may also be present. The best-known structure of a transmembrane protein is that of the 248-residue bacteriorhodopsin. It consists of seven helical segments that span the plasma membrane (Fig. 23 5) and serves as a light-activated proton pump. Other proteins with similar structures act as hormone receptors in eukaryotic membranes. A seven-helix protein embedded in a membrane is depicted in Fig. 8-5 and also, in more detail, in Fig. 11-6. 

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