Surface-exposed loop regions

Fig. 2. Sequence content of spa repeats at nucleotide and amino acid levels. Repeats differing at the nucleotide level may be identical at the amino acid level. Identical codons are depicted using the same colors equivalent codons corresponding to synonymous mutations are colored with similar hues. The translated sequence reveals a conserved octapeptide motif comprised almost exclusively of charged amino acids and is suggestive of a surface-exposed loop region with repeating turns. Repeats Tl-Zl bear a single codon insertion between positions 1 and 2 most common spa types begin with one of these repeats, such that the initial repeat is usually 27-bp in length (vs. 24-bp).

The complex with ADP and aluminium fluoride is thought to resemble the early ADP.Pi state immediately after hydrolysis, whereas the complex with ADP and vanadate may represent a late state in which the phosphate (mimicked by vanadate) has moved quite a long distance (15 A) from the active center to the surface of the motor domain. There it is fixed by two hydrogen bonds to the solvent exposed tips of the switch-1 loop region (L9) at one side and the switch-2 loop (Lll) at the other side. 

Love and Stroud, 1986). All postsynaptic neurotoxins are similar in their overall folding, but differ in details such as the extent of secondary structure and the position of an invariant side-chain. Recently, the NMR three dimensional structure of cobrotoxin in solution has been determined (Yu et al, 1993). The mean solution structure was compared with the X-ray crystal structure of homologous protein erabutoxin b which has been solved to a resolution of 1.4 A (Low and Corifield, 1986) (Fig. 3). This yielded information that both structures resemble each other except at the exposed loops and surface regions, where the solution structure reveals the higher flexibility in its conformation. 

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