Polypeptide chains cross linked

The molecular weight of a protein can be determined by SDS-polyacrylamide gel electrophoresis or by sedimentation equilibrium. Which method would you use to determine the molecular weight of a protein containing four subunits, each consisting of two polypeptide chains cross-linked by two disulfide bridges Explain your answer. 

Secondary structures can take two forms an a-helix is a spiral shape, whereas the pleated sheet looks like a piece of paper folded many times. The parallel polypeptide chains cross-linked by hydrogen bonds form an extremely tough structure. Silk is an example (Hale et al., 1995). 

Anticholinesterase-type neurotoxin has 57-60 amino acids in a single polypeptide chain, cross-linked by three disulfide bonds. The two-dimensional structure of fasciculin 2 from dendroaspis venom is shown in Fig. 15. Fasciculin 2 is identical with toxin F7 isolated by Viljoen and Botes (1973). Similarly, toxins C and D from D. polylepis polylepis venom... 

Photographic material containing gelatin can be hardened during manufacture the process involves cross-linking between the gelatine polypeptide chains induced by hardener. 

A polymer is a macromolecule that is constructed by chemically linking together a sequent of molecular fragments. In simple synthetic polymers such as polyethylene or polystyrer all of the molecular fragments comprise the same basic unit (or monomer). Other poly me contain mixtures of monomers. Proteins, for example, are polypeptide chains in which eac unit is one of the twenty amino acids. Cross-linking between different chains gives rise to j-further variations in the constitution and structure of a polymer. All of these features me affect the overall properties of the molecule, sometimes in a dramatic way. Moreover, or... 

The differences in the amino acid chemistry of the hide coUagen and the hair keratin are the basis of the lime-sulfide unhairing system. Hair contains the amino acid cystine. This sulfur-containing amino acid cross-links the polypeptide chains of mature hair proteins. In modem production of bovine leathers the quantity of sulfide, as Na2S or NaSH, is normally 2—4% based on the weight of the hides. The lime is essentially an unhmited supply of alkah buffered to pH 12—12.5. The sulfide breaks the polypeptide S—S cross-links by reduction. Unhairing without sulfide may take several days or weeks. The keratin can be easily hydrolyzed once there is a breakdown in the hair fiber stmcture and the hair can be removed mechanically. The coUagen hydrolysis is not affected by the presence of the sulfides (1—4,7). 

If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

Collagen triple helices are stabilized by hydrogen bonds between residues in dijferent polypeptide chains. The hydroxyl groups of hydroxyprolyl residues also participate in interchain hydrogen bonding. Additional stability is provided by covalent cross-links formed between modified lysyl residues both within and between polypeptide chains. 

Tanford (1968) reviewed early studies of protein denaturation and concluded that high concentrations of Gdm-HCl and, in some cases, urea are capable of unfolding proteins that lack disulfide cross-links to random coils. This conclusion was largely based on intrinsic viscosity data, but optical rotation and optical rotatory dispersion (ORD) [reviewed by Urnes and Doty (1961) ] were also cited as providing supporting evidence. By these same lines of evidence, heat- and acid-unfolded proteins were held to be less completely unfolded, with some residual secondary and tertiary structure. As noted in Section II, a polypeptide chain can behave hydrodynamically as random coil and yet possess local order. Similarly, the optical rotation and ORD criteria used for a random coil by Tanford and others are not capable of excluding local order in largely unfolded polypeptides and proteins. The ability to measure the ORD, and especially the CD spectra, of unfolded polypeptides and proteins in the far UV provides much more incisive information about the conformation of proteins, folded and unfolded. The CD spectra of many unfolded proteins have been reported, but there have been few systematic studies. 

The transglutaminases are calcium-dependent enzymes that catalyse the cross-linking of proteins by promoting the formation of isopeptide bonds between the /-carboxyl group of a glutamine in one polypeptide chain and the e-amino group of a lysine in the second (Greenberg et al., 1991). These... 

The basic structure of an immunoglobulin molecule, such as the major serum antibody IgG, consists of four polypeptide chains two identical light chains (molecular weight around 25 000 daltons) and two identical heavy chains (with a molecular weight around 50 000 daltons), cross-linked by disulfide bonds to form Y-shaped molecules with two flexible arms (Fig. 11.2). The binding sites are located on the arms and vary from one molecule to another (variable region) [22b]. 

This covalent bonding arises as a result of biochemical oxidation of the thiol groups in two cysteine residues, and it may also be achieved chemically with the use of mild oxidizing agents. This modification of thiol groups may thus loop a polypeptide chain or cross-link two separate chains. It also significantly modifies the properties of a... 

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