Collagen disulfide bonds

Yang, J. and Kramer, J.M. (1994) In vitro mutagenesis of Caenorhabditis elegans cuticle collagens identifies a potential subtilisin-like protease cleavage site and demonstrates that carboxyl domain disulfide bonding is required for normal function but not assembly. Molecular and Cellular Biology 14, 2722-2730. 

The second class of AChEs exists as heteromeric assemblies of catalytic and structural subunits. One form consists of up to 12 catalytic subunits linked by disulfide bonds to filamentous, collagen-containing structural subunits. These forms are often termed asymmetric, since the tail unit imparts substantial dimensional asymmetry to the molecule. The collagenous tail unit links by disulfide bonding at its proline rich N-terminus through a coiled coil arrangement to the C-terminus of two of the catalytic subunits [30]. The tail unit associates with the basal lamina of the synapse rather than the plasma membrane. 

Fig. 2.4. Schematic model of the molecular polymorphism of acetylcholinesterase and cholinesterase [110][112a]. Open circles represent the globular (G) catalytic subunits. Disulfide bonds are indicated by S-S. The homomeric class exists as monomers (Gl), dimers (G2), and tetramers (G4) and can be subdivided into hydrophilic (water-soluble) and amphiphilic (membrane-bound) forms. The G2 amphiphilic forms of erythrocytes have a glycophospholipid anchor. The heteromeric class exists as amphiphilic G4 and as asymmetric forms (A) containing one to three tetramers. Thus, heteromeric G4 forms found in brain are anchored into a phospholipid membrane through a 20 kDa anchor. The asymmetric A12 forms have three hydrophilic G4 heads linked to a collagen tail via disulfide bonds.

Type I collagen molecules are heterotrimeric molecules with an a chain composition of two al (I) chains and one o 2(I) chain from the COLlAl and COLl A2 genes, respectively. This a chain composition is written as [a 1(1)] 20 2 (I). A type I collagen molecule is synthesized as a procollagen type I molecule. The N- and C-terminal propeptides are cleaved as part of the processing in tissues and are called N- and C-propeptides, respectively. The human pro-o (I) and pro-o 2(I) have 246 and 247 residues in their C-propeptides, respectively. The C-propeptide contains interchain disulfide bonds, which are important for the stabilization of the three a chains in the endoplasmic reticulum (ER). 

Figure 6 Model of type VI collagen assembly. Two type VI collagen molecules assemble with 30 nm overlap " with two pairs of disulfide bonds between cysteines, one in collagenous domain and another in the C-terminal globular domain.Two dimers form a tetramer with disulfide bonds presumably in the a3(VI) chains. The tetramers assemble into the long beaded filamentous structure with 105nm periodicity.

Prolyl 4-hydroxylation is the most abundant posttranslational modification of collagens. 4-Hydroxylation of proline residues increases the stability of the triple helix and is a key element in the folding of the collagen triple helix. " In vertebrates, almost all the Yaa position prolines of the Gly-Xaa-Yaa repeat are modified to 4(I( )-hydroxylproline by the enzyme P4H (EC 1.14.11.2), a member of Fe(II)- and 2-oxoglutarate-dependent dioxygenases. This enzyme is an 0 2/ b2-type heterotetramer in which the / subunit is PDI (EC 5.3.4.1), which is a ubiquitous disulfide bond catalyst. The P4H a subunit needs the 13 subunit for solubility however, the 13 subunit, PDI, is soluble by itself and is present in excess in the ER. Three isoforms of the a subunit have been identified and shown to combine with PDI to form [a(I)]2/ 2) [< (II)]2/32> or [a(III)]2/32 tetramers, called the type... 

Fibronectins are typical representatives of adhesive proteins. They are filamentous dimers consisting of two related peptide chains (each with a mass of 250 kDa) linked to each other by disulfide bonds. The fibronectin molecules are divided into different domains, which bind to cell-surface receptors, collagens, fibrin, and various proteoglycans. This is what gives fibronectins their molecular glue" characteristics. 

The primary level of structure in a protein is the linear sequence of amino acids as joined together by peptide bonds. This sequence is determined by the sequence of nucleotide bases in the gene encoding the protein (see Topic HI). Also included under primary structure is the location of any other covalent bonds. These are primarily disulfide bonds between cysteine residues that are adjacent in space but not in the linear amino acid sequence. These covalent cross-links between separate polypeptide chains or between different parts of the same chain are formed by the oxidation of the SH groups on cysteine residues that are juxtaposed in space (Fig. 4). The resulting disulfide is called a cystine residue. Disulfide bonds are often present in extracellular proteins, but are rarely found in intracellular proteins. Some proteins, such as collagen, have covalent cross-links formed between the side-chains of Lys residues (see Topic B5). 

The strength and rigidity of a collagen fiber is imparted by covalent cross-links both between and within the tropocollagen molecules. As there are few, if any, Cys residues in the final mature collagen, these covalent cross-links are not disulfide bonds as commonly found in proteins, but rather are unique cross-links formed between Lys and its aldehyde derivative allysine. Allysine residues are formed from Lys by the action of the monooxygenase lysyl oxidase... 

Fig. 6. Loop structures in the NCI domain of collagen IV chains. The beginning portion of the triple-helical domain is shown as a jagged line labeled N. The NCI domain contains two repeating structures, each comprising a large loop, followed by a small loop, and then another large loop. The location of the intrachain disulfide bonds has been deduced (Sieboldt el al., unpublished).

Alkali has long been used on proteins for such processes as the retting of wool and curing of collagen, but more recently it has received interest from the food industry. Alkali can cause many changes such as the hydrolysis of susceptible amide and peptide bonds, racemization of amino acids, splitting of disulfide bonds, beta elimination, and formation of cross-linked products such as lysinoalanine and lanthionine. 

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