Light chains

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

Figure 14.14 Sci ematic diagram of the myosin molecule, comprising two heavy chains (green) that form a coiled-coil tail with two globular heads and four light chains (gray) of two slightly differing sizes, each one bound to each heavy-chain globular head.

Figure 14.15 Stmcture of the SI fragment of chicken myosin as a Richardson diagram (a) and a space-filling model (b). The two light chains are shown in magenta and yellow. The heavy chain is colored according to three proteolytic fragments produced by trypsin a 25-kDa N-terminal domain (green) a central 50-kDa fragment (red) divided by a cleft into a 50K upper and a 50K lower domain and a 20-kDa C-terminal domain (blue) that links the myosin head to the coiled-coil tail. The 50-kDa and 20-kDa domains both bind actin, while the 25-kDa domain binds ATP. [(b) Courtesy of 1. Rayment.]...

The basic structure of all immunoglobulin (Ig) molecules comprises two identical light chains and two identical heavy chains linked together by disulfide bonds (Figure IS.2a). There are two different classes, or isotypes, of light chains, X and k, but there is no known functional distinction between them. Heavy chains, by contrast, have five different isotypes that divide the immunoglobulins into different functional classes IgG, IgM, IgA, IgD, and IgE, each with different effector properties in the elimination of antigen... 

Figure 15.2b). Each class of heavy chains can combine with either of the two different classes of light chains. 

In this chapter we will discuss immunoglobulins of the IgG class, which is the major type of immunoglobulin in normal human serum, and which has the simplest structure. Each chain of an IgG molecule is divided into domains of about 110 amino acid residues. The light chains have two such domains, and the heavy chains have four. 

The most remarkable feature of the antibody molecule is revealed by comparing the amino acid sequences from many different immunoglobulin IgG molecules. This comparison shows that between different IgGs the amino-terminal domain of each polypeptide chain is highly variable, whereas the remaining domains have constant sequences. A light chain is thus built up from one amino-terminal variable domain (Vl) and one carboxy-terminal constant domain (Cl), and a heavy chain from one amino-terminal variable domain (Vh), followed by three constant domains (Chi, Ch2. and Chs). 

The antigen-binding site is formed by close association of the hypervariable regions from both heavy and light chains... 

Figure 15.12 Schematic diagram of the barrel arrangement of four p strands from each of the variable domains in Fab. The six hypervariable regions, CDR1-CDR3 from the light chain (L1-L3) and from the heavy chain (H1-H3), are at one end of this barrel. (From J. Novotny et al., /. Biol. Chem. 2S8 14433-14437, 1983.)...

Figure 15.16 Detailed views of the environment of Gin 121 in the lysozyme-antilysozyme complex. Gin 121 in lysozyme is colored green both in the space-filling representation to the left and in the ball and stick model to the right. This side chain of the antigen fits into a hole between CDR3 regions of both the heavy (Tyr 101) and the light (Trp 92) chains as well as CDRl from the light chain (Tyr 32). (After A.G. Amit et al.. Science 233 747-753, 1986.)...

Figure 15.17 The three-dimensional structure of an intact IgG. Hinge regions connecting the Fab arms with the Fc stem are relatively flexible, despite the presence of disulfide bonds in this region linking the heavy and light chains. Carbohydrate residues that bridge the two Ch2 domains are not shown. (Courtesy of A. McPherson and L. Harris, Nature 360 369-372, 1992, by copyright permission of Macmillan Magazines Limited.)...

The genes for MHC molecules, unlike immunoglobulin genes, do not undergo rearrangements to create structural diversity. The Pzm light chain is invariant, but the class I MHC heavy chain is the most genetically polymorphic... 

IgG antibody molecules are composed of two light chains and two heavy chains joined together by disulfide bonds. Each light chain has one variable domain and one constant domain, while each heavy chain has one variable and three constant domains. All of the domains have a similar three-dimensional structure known as the immunoglobulin fold. The Fc stem of the molecule is formed by constant domains from each of the heavy chains, while two Fab arms are formed by constant and variable domains from both heavy and light chains. The hinge region between the stem and the arms is flexible and allows the arms to move relative to each other and to the stem. 

The constant domain has a stable framework structure composed of two antiparallel sheets comprising seven p strands, four in one sheet and three in the other. The variable domains have a similar framework structure but comprising nine p strands, five in one sheet and four in the other. The three hypervariable regions are in loops at one end of the variable domain. The variable domains from the heavy and light chains associate through their five-stranded p sheets to form a barrel with the hypervariable loop regions from both domains close together at the top of the barrel. 

Figure 17.2 An example of prediction of the conformations of three CDR regions of a monoclonal antibody (top row) compared with the unrefined x-ray structure (bottom row). LI and L2 are CDR regions of the light chain, and HI is from the heavy chain. The amino acid sequences of the loop regions were modeled by comparison with the sequences of loop regions selected from a database of known antibody structures. The three-dimensional structure of two of the loop regions, LI and L2, were in good agreement with the preliminary x-ray structure, whereas HI was not. However, during later refinement of the x-ray structure errors were found in the conformations of HI, and in the refined x-ray structure this loop was found to agree with the predicted conformations. In fact, all six loop conformations were correctly predicted in this case. (From C. Chothia et al.. Science 233 755-758, 1986.)...

TSK-GEL SW columns have also been used to isolate heavy and light chains from immunoglobulins. Kast et al. (7) reported how these chains were desalted and isolated on a 21.5-mm i.d. X 60-cm TSK-GEL G4000SW semipreparative column using a 0.1 trifluoroacetic acid (TEA)/40% acetonitrile mobile phase. 

Kast, E., Pathmanbhan, N., Wong, J., O Connor, B., and Klein, M. (1996). Poster presented at Protein Society 1996, Preparative Isolation of Immunoglobulin Heavy and Light Chains by Size-Exclusion Chromatogtaphy . 

Myosin light-chain kinase (MLCK) —ia[Pg.467]

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