Complementarity variables

The anticodon region consists of seven nucleotides, and it recognizes the three-letter codon in mRNA (Figure 38-2). The sequence read from the 3 to 5 direction in that anticodon loop consists of a variable base-modified purine-XYZ-pyrimidine-pyrimidine-5h Note that this direction of reading the anticodon is 3 " to 5 whereas the genetic code in Table 38—1 is read 5 to 3 since the codon and the anticodon loop of the mRNA and tRNA molecules, respectively, are antipar-allel in their complementarity just like all other inter-molecular interactions between nucleic acid strands. 

The differences in sequence within the variable region produce a practically unlimited number of possible three-dimensional arrangements for the amino acid side chains to form the complementarity-determining region (CDR), which actually binds to the antigen. 

It is, perhaps, less known that the concepts of complementarity and indeterminacy also arise naturally in the theory of Brownian motion. In fact, position and apparent velocity of a Brownian particle are complementary in the sense of Bohr they are subject to an indeterminacy relation formally similar to that of quantum mechanics, but physically of a different origin. Position and apparent velocity are not conjugate variables in the sense of mechanics. The indeterminacy is due to the statistical character of the apparent velocity, which, incidentally, obeys a non-linear (Burgers ) equation. This is discussed in part I. 

Wu, T.T. Rabat, E.A. (1970). An analysis of the sequences of the variable regions of Bence Jones proteins and myeloma light chains and their implications for antibody complementarity. J. Exp. Med. 132,211-250. 

Fig. 12.7 Space-filling model of an IgG molecule. VL and VH variable domain light and heavy chain CL and CH constant domain light and heavy chain LCDR and HCDR complementarity-determining regions on VL and VH domains.

---