Polypeptides strands

P Sheet A parallel or antiparallel arrangement of the polypeptide chain. Hydrogen bonds are formed between the two (or more) polypeptide strands. 

The extracellular domain is linked to the transmembrane domain, which may be formed by up to 12 polypeptide strands that pass through the membrane. 

Breaking Disulfide Bonds Disulfide bonds interfere with the sequencing procedure. A cystine residue (Fig. 3-7) that has one of its peptide bonds cleaved by the Edman procedure may remain attached to another polypeptide strand via its disulfide bond. Disulfide bonds also interfere with the enzymatic or chemical cleavage of the polypeptide. Two approaches to irreversible breakage of disulfide bonds are outlined in Figure 3-26. 

Was this youT answeT A codon is a triplet of mRNA nucleotides holding instructions for the building of a polypeptide strand, and so this sequence contains seven codons. The first andlast are start/stop instructions, however, and so the number of amino acids coded for is only five. The sequence is Leu-Lys-Ser-Gl n-Al a. 

Figure 25-17 Representation of the quaternary structure of a-keratin showing (a) three a-helical polypeptide strands coiled into a rope and (b) eleven units of the three-stranded rope arranged to form one microfibril...

The principal protein of skin and connective tissue is called collagen and is primarily constituted of glycine, proline, and hydroxyproline. Collagen is made up of tropocollagen, a substance with very long and thin molecules (14 X 2900 A, MW about 300,000). Each tropocollagen molecule consists of three twisted polypeptide strands. When collagen is boiled with water, the... 

The second major regular, repeating structure, the /3 structure, differs from the a helix in that the polypeptide chains are almost completely extended, as in Fig. 4-7(a), and hydrogen bonding occurs between polypeptide strands, rather than within a single strand, as shown in Fig. 4-7(c). 

Pleated sheet structures are parallel or antiparallel. In the local minimum in the Ramachandran qt/y/ plot (Fig. 19.3) of y3-pleated sheet structures, two configurations are possible, with parallel and antiparallel orientation of the polypeptide strands (Fig. 19.6). The strands are linked by mferchain N-H 0=C hydrogen bonds, which run both ways between the strands and produce a characteristically different pattern in parallel and antiparallel sheets. It is a particular stereochemical feature of the /7-pleated sheets that amino acid side-chains point alternately up and down, and adjacent side-chains interact sterically to produce a right-handed twist [597, 5981 (see Fig. 19.7 a). The regular pattern of a /7-sheet can be interrupted locally by insertion of an extra amino acid, giving rise to a so-called /7-bulge [599]. 

Figure 3 Topology diagram showing the connectivity of the eight extended polypeptide strands which make up the /3-barrel in type 1 copper proteins. The locations of the common copper hgands and the carbonyl oxygen in azurin are shown as filled circles and squares...

Beta Sheets Are Stabilized by Hydrogen Bonding Between Polypeptide Strands... 

As a consequence of the individual order of the amino acids and the conformation of the ensuing polypeptide strands (the primary and secondary structures) the three-dimensional structure of each molecule (its tertiary structure) is formed. The bulk of this Chapter is focussed on this aspect of molecular structure. Some comments on quaternary structure and protein crystallisation form a shorter afterword. 

A / sheet is formed when individual polypeptide strands line up side by side and form hydrogen bonds of the type C=0 H-N. 

One other important principle to be learned from Nature is that when the structural dimensions of the architectures enter the multi-micrometer domain, different levels of organization are involved, e.g., as in collagen, which consists of polypeptide strands that are organized in triple helices (tropocollagen) that assemble to form fibrils and ultimately generate the... 

The receptor complex appears to be formed of these polypeptide strands into a rosetteshaped aggregate surrounding an ion pore visible at 250,000 magnification. The structure is embedded in the postsynaptic membrane and has been shown to have permease functions by demonstrating that the passage of cations was facilitated by ACh and blocked by the neuromuscular blocking agent, tubocurarine. 

The 3-pleatBd sheet is described as parallel if the polypeptide strands run in the same direction (as defined by their amino and carboxy terminals) and anti-parallel if they run in opposite directions. Antiparallel strands are often the same polypeptide chain folded back on itself, with simple hairpin turns or long runs of polypeptide chain connecting the strands. The amino acid side chains of each polypeptide strand alternate between extending above and below the plane of the 3-sheet (see Fig. 7.6). Parallel sheets tend to have hydrophobic residues on both sides of the sheets antiparallel sheets usually have a hydrophobic side and a hydrophilic side. Frequently, sheets twist in one direction. 

Sign in atwww.thomsonedu.com/login and explore a Biochemistry Interactive tutorial on the features of/3-turns and how they change the direction of a polypeptide strand. 

---