H and L chains

The polypeptide chains are held together by disulfide bonds between the H and L chains within each half-molecule and between the H chains that join at the hinge region. 

Kishimoto, T., Okajima, H., Okumoto, T., et al. (1989). Nucleotide sequences of the cDNAs encoding the V regions of H and L chains of a human monoclonal antibody with broad reactivity to a malignant tumor. Nucl. Acids Res., 17, 4385-4385. 

Ligate H and L chain cDNAs into X expression vector... 

Infect E. coli H and L chains are produced in infected cells... 

H and L chain-encoding cDNAs from reactive antibody are cloned... 

Dissociation of the phytohemagglutinin subunits was accompanied by a complete release of Mn2+ from the protein.444 Carbohydrate and Ca2+ remained bound to both subunits. Very low hemagglutinating activity was associated with the heavy subunit the light subunit was inactive. Addition of Mn2+ and Ca2+ to a mixture of H and L chains failed to restore hemagglutinating activity.444... 

Amino acid analysis of the isolated subunits showed that only the H chains contained cysteine (one residue per molecule).444 Unfortunately, this amino acid analysis did not include methionine, reported absent by one group441 and present by a second group.143 End-group analysis showed N-terminal valine and threonine for the H and L chains, respectively. Treatment with carboxypeptidase gave serine as the C-terminal amino acid for both subunits. 

Mammalian ferritins are heteropolymers of H- and L-chains. These subunits are very closely related, with an a-carbon rmsd of 0.5 A and 55% sequence identity conservation of primary sequence rises to 79% when considering those residues responsible for intersubunit interactions. Subunit assembly appears to take place via partially structured monomers associating to form fully structured homodimers, which then aggregate further. Upon chemical denaturation and refolding, heterodimers are rarely observed. ... 

Ferritins are a ubiquitous class of robust proteins involved in the biomineralization of iron oxide particles. Structurally, the protein shell is assembled from 24 peptide subunits (4 helical bundles) of repeating H and L chains which form the fundamental spheroidal shape provide a large spatially arranged compartment (9.0-nm diameter) for mineralization and generate hydrophilic and hydrophobic channels with openings of 0.5 nm that lead directly to the cavity... 

Very low-angle X-ray data (26 A resolution) of horse spleen apoferritin fit approximately the Fourier transform of a uniform spherical shell with inner and outer diameters of 76 and 122 A (92, 95). Low-angle difference X-ray data for ferritin and apoferritin indicate iron cores of high scattering power that are approximately spherical (d = 78 A) (92). Cubic crystal point symmetry shows that ferritin molecules are composed of 24 structurally equivalent subunits related by 432 symmetry, there being one polypeptide chain per asymmetric unit. In mixed H and L chain copolymers, the apparent structural equivalence must be statistical, although very similar chain conformations are expected. 

The interaction of CNTs with membrane bilayers has been studied mainly with model membrane systems, and limited data have been obtained in vivo. Available evidence indicates that at low pH TeTx and BoNTs undergo a conformational change from a water soluble "neutral" form to an "acidic" form, the latter characterized by the exposure of hydrophobic segments. This increase in hydrophobicity allows penetration of both the H and L chains into the hydrocarbon core of the lipid bilayer (Montecucco etal., 1994). Following this low pH-induced membrane insertion, TeTx and BoNTs form ion channels in planar lipid bilayers (Beise et al., 1994 Montecucco et al., 1994). These channels are cation-selective, have few tens of pS conductance and are per-... 

Reduction of the toxin and translocation of the L chain into the cytoplasm To gain access to the cytoplasm, the L chain needs to cross the membrane of the endocytic compartment. For this translocation, the H chain is required, probably by forming a proteinaceous translocation complex in the membrane that exposes (and possibly releases) the L chain to the cytoplasm. In the reductive intracellular environment, the disulfide bond linking the H and L chains is reduced (Kistner and Habermann, 1992). 

Ferritins can store excess iron and release it when required for cellular processes. In addition, neuromelanin (NM), which is an organic polymer consisting of dihydroxyindole and benzothiazine units which are products of dopamine metabolism, is also present and is able to bind a number of metals, e.g., copper and iron. In oligodendrocytes, iron is bound to both H- and L-chain ferritin, in microglia to L-ferritin, while neurons contain mostly neuromelanin. In contrast, astrocytes contain hardly any ferritin. As to the movement of iron between different brain regions, this in the main remains unclear. It is thought that transferrin and ferritin may be important, since mRNA receptors for these iron proteins are detectable in grey matter and white matter, respectively. The fate of non-transferrin-bound iron, which may cross the BBB, remains unclear. 

Figure 8-2 Posttranscriptional regulation by IRE/IRP interactions. In iron-replete cells, IRPs do not bind to IREs. Iron starvation induces IRPs to bind to their ligands, resulting in stabilization of transferrin receptor mRNA and translational inhibition of the mRNAs encoding ferritin H- and L-chain), eALAS and mitochondrial aconitase.

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