L-chains

The reaction center is built up from four polypeptide chains, three of which are called L, M, and H because they were thought to have light, medium, and heavy molecular masses as deduced from their electrophoretic mobility on SDS-PAGE. Subsequent amino acid sequence determinations showed, however, that the H chain is in fact the smallest with 258 amino acids, followed by the L chain with 273 amino acids. The M chain is the largest polypeptide with 323 amino acids. This discrepancy between apparent relative masses and real molecular weights illustrates the uncertainty in deducing molecular masses of membrane-bound proteins from their mobility in electrophoretic gels. 

Alk l chain. The increase in the alkyl chain length and linearity improves the efficiency of the plasticizer and the low-temperature flexibility of the plasticized polymers. 

The most ingenious exocytosis toxins, however, come from the anaerobic bacteria Clostridium botulinum and Clostridium tetani. The former produces the seven botulinum neurotoxins (BoNTs) A-G the latter produces tetanus neurotoxin (TeNT). All eight toxins consist of a heavy (H) chain and a light (L) chain that are associated by an interchain S-S bond. The L-chains enter the cytosol of axon terminals. Importantly, BoNT L-chains mainly enter peripheral cholinergic terminals, whereas the TeNT L-chain mainly enters cerebral and spinal cord GABAergic and glycinergic terminals. The L-chains are the active domains of the toxins. They are zinc-endopeptidases and specifically split the three core proteins of exocytosis, i.e. the SNAREs (Fig. 1 inset). Each ofthe eight toxins splits a... 

Is circular, double-stranded, and composed of heavy (H) and a light (L) chains or strands. 

Collagens Collagen type 1 Approximately 90% of total bone protein. Composed of two a1(l) and one a2(l) chains. 

Figure 49-4. Diagram of a myosin moiecuie showing the two intertwined a-heiices (fibrous portion), the giobuiar region or head (G),the iight chains (L), and the effects of proteoiytic cieavage by trypsin and papain. The giobuiar region (myosin head) contains an actin-binding site and an L chain-binding site and aiso attaches to the remainder of the myosin moiecuie.

S-1 (molecular mass approximately 115 kDa) does exhibit ATPase activity, binds L chains, and in the absence of ATP will bind to and decorate actin with arrowheads (Figure 49-5). Both S-1 and HMM exhibit ATPase activity, which is accelerated 100- to 200-fold by complexing with F-actin. As discussed below, F-actin greatly enhances the rate at which myosin ATPase releases its products, ADP and Pj. Thus, although F-actin does not affect the hydrolysis step per se, its ability to promote release of the products produced by the ATPase activity greatly accelerates the overall rate of catalysis. 

The L chains and H chains are synthesized as separate molecules and are subsequently assembled within the B cell or plasma cell into mature immunoglobulin molecules, all of which are glycoproteins. 

Table 6.1 Amino-acid sequence alignment of four mammalian ferritins (Horse L chain, HoL Human L chain, HuL Human H chain, HuH Rat H, RaH) and of one of the ferritins, FTN, and the bacterioferritin, BFR of...

Figure 6.1 Schematic representation of human isoferritins of different subunit composition. Each ferritin subunit is represented as a sausage and subunits are packed in a symmetrical shell. Twelve of the 24 subunits are visible, with H-chain subunits stippled and L-chain subunits plain. Homopolymers of H-chain and L-chain subunits are at the top and bottom of the figure respectively. The sources of various ferritins are listed in the right hand column. Reprinted from Harrison and Arosio, 1996. Copyright (1996), with permission from Elsevier Science.

This is done a contre coeur, since the L-chain sequence was the first to be established. However, it makes little sense to refer to the amino terminus of H-chain ferritins as —1, —2, etc. 

The arrangement of the 24 subunits of the apoferritin molecule in their 432 symmetry viewed down a fourfold axis is presented in Figure 6.3. Also included in Figure 6.3 is a labelling scheme of symmetry related subunits and a representation of the subunit as a ribbon diagram of the -carbon backbone. Of the 174 amino-acid residues of the L-chain 140 (80 %) are found in five a-helices. Each of the 24 subunits consists... 

Purified C5-1 has been obtained from alfalfa leaf extracts by affinity chromatography on either a human IgG-Sepharose column or a Streamline rProtein A-Sepharose column. Interestingly, the purified product obtained with these two methods differed significantly. As shown in Fig. 1.5 a, the antibody fraction obtained from the human IgG column contained a mixture of different intermediate assembly forms of the heavy (H) and light (L) chains, ranging from H2 to the fully assembled H2L2 form. 

Inoue, S., Tanaka, K., Arisaka, F., Kimura, S., Ohtomo, K., and Mizuno, S. (2000a). Silk fibroin of Bombyx mori is secreted, assembling a high molecular mass elementary unit consisting of H-chain, L-chain, and P25, with a 6 6 1 molar ratio./. Biol. Chem. 275, 40517-40528. 

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