Subunits, disulfide-linked

Two protein hormones, inhibin and activin, have been identified in gonadal tissue. Inhibin has been isolated from ovarian foUicular fluid and found to inhibit pituitary secretion of FSH. Inhibin is a glyocoprotein heterodimer consisting of two disulfide-linked subunits, a and P two types of P-subunit,... 

T-cell antigen receptors belong to a separate family, which is composed of eight noncovalently bound trans-membrane subunits. The central ligand-binding element is a disulfide linked a,/1-heterodimer linked to the membrane by a short transmembrane sequence. The dimeric, variable subunits are the cellular equi-... 

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.

Fig. 16. Schematic depiction of the murine hemopexin receptor. The results of crosslinking studies (163) suggested that the murine receptor was composed of two subunits (a and p). The larger (65 kDa) is designated as a and is disulfide-linked to the smaller P subunit, which contains a free sulfhydryl. Hemopexin (which has no free sulfhydryl groups) can be crosslinked to the a-subunit but may also bind to the p-subunit.

Signaling by an activated T-cell receptor is quite complex. The ITAMs are sites of tyrosine phosphorylation by kinases of the Src family.335 336 Another tyrosine kinase, Zap-70 (Fig. 31-15), associates with the C-terminal tails of the disulfide-linked dimer of subunits C- It recognizes the phosphotyrosines groups via its SH2 domains (see Fig. 7-30). Zap-70 appears to act... 

Sialidase is a homotetramer with C4 symmetry composed of identical disulfide-linked subunits (Fig. 17.6a [62]). Each monomer is a glycosylated polypeptide with six p-sheets assuming the appearance of six blades of a propeller [6, 64] with a right-handed twist. The catalytic site is observed to be at the center of the sixfold pseudosymmetry axis, which passes through the center of each monomer and relates the six p-sheets to each other (Fig. 17.6b [63]) [64, 65], The active site contains a large number of conserved amino acid residues, which are involved in binding to sialic acid in the substrate sialoglycoconjugate [64],... 

On the basis of these data, the following model of the subunit structure is proposed the snail hemagglutinin consists of 6 identical, polypeptide chains (subunits), each containing one intrachain disulfide bond and a carbohydrate binding-site. Furthermore, two subunits are linked by an intrachain, disulfide bond to form subunit dimers of molecular weight 26,000, and three dimers (mol. wt. 26,000) are held together by noncovalent interactions.569... 

GPIIb has a molecular weight of 136,000 daltons. It is composed of a larger alpha (125,000 daltons) and a smaller beta (23,000 daltons) subunit, linked by a single disulfide bond (37). The beta subunit serves to anchor GPIIb in the platelet membrane. The disulfide linked alpha subunit is entirely extracellular. 

Figure 6. Proposed quaternary structures of rhSCF dimer and disulfide-linked dimer. A, SDS-dissociable rhSCF dimer with topology similar to M-CSF. B, disulfide-linked dimer having all disulfides at the dimer interface. C, disulfide-linked dimer containing A and D helices swapped between subunits (distinguished as shaded and unshaded helices). The four helical structure (A-D helices) was derived from that proposed by Bazan (27).

The glycoproteins (ricin) are poorly absorbed from the gastrointestinal tract however, once absorbed, they most likely follow a distribution pattern similar to that of albumin. Many cell surfaces contain receptors specific for the ricin molecules. This molecule consists of two subunits, A and B, bound by a disulfide link. When this link is broken, the B subunit binds to galactose-containing receptors in the cell wall and is transported intracellularly. The A subunit inhibits protein synthesis. The liver, spleen, adrenal cortex, and bone marrow are the primary sites of distribution. The biotransformation and elimination of toxalbumins are poorly understood. The elimination half-life in one patient was 2 days. The reported disappearance of ricin from the plasma is according to first-order kinetics when... 

IL-12 is a 75-kDa heterodimer with two disulfide-linked subunits (p35 and p40), and is produced by macrophages (525), dendritic cells (526), and airway epithelial cells (527). Expression of IE-12 receptor (IL-12R) is limited to activated T- and NK cells. Functional IL-12R has pi and /32 subunits with p40 ligand segment binding primarily to pi, whereas the p35 ligand unit associates with the p2 chain of the receptor (528). The P2 unit appears to be responsible for signal transduction (529). 

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