Baboon a-lactalbumin

In any event it was the analysis of baboon a-lactalbumin crystals for which the first X-ray crystal structure was produced, initially at 0.6 nm (6 A) and 0.45 nm (4.5 A) (Phillips etal., 1987 Smith etal., 1987). More recently, the structure has been refined at 0.17-nm (1.7-A) resolution, enabling comparison with the high-resolution c-type lysozyme structure (Acharya et al., 1989) (see Fig. 7). 

Fig. 7. The tertiary structure of (a) baboon a-lactalbumin and (b) domestic hen egg-white lysozyme. (Reproduced with permission from Acharya et ai, 1989 based on a program of J. P. Priestle.)...

Subsequently, the baboon a-lactalbumin structure was refined at 1.7-A resolution by Acharya et al. (1989). Using the structure of domestic hen egg white lysozyme as the starting model, preliminary refinement was made using heavily constrained least-squares minimization in reciprocal space. Further refinement was made using stereochemical restraints at 1.7-A resolution to a conventional crystallographic residual of 0.22 for 1141 protein atoms. 

The human a-lactalbumin amino acid sequence was used in the refinement since the baboon sequence has not been determined, although it was known from the unpublished work by R. Greenberg to be close to the human sequence. However, it became evident in the course of the X-ray work that there were eight sequence changes in baboon a-lactalbumins (see Section VII,B). 

There are similarities in the helices and /3 sheets between baboon a-lactalbumin and hen egg-white lysozyme, as summarized in Table V. However, there are important differences, for example, in hen egg-white lysozyme residues 41—60 form an irregular antiparallel j8-pleated sheet in this protein a residue is deleted at position 48 (human lysozyme numbering), but two residues are deleted in a-lactalbumin at positions 47 and 48 (human lysozyme numbering). Residue 47 is the most exposed to solvent in the hen egg-white lysozyme and forms part of the irregular 0 turn. These residues occur in a -pleated sheet and the deletions are accommodated with minimal disruption to the pleated sheet (see the comparison in Acharya et al., 1989). 

Comparison of Structural Elements for Domestic Hen Egg-White Lysozyme and Baboon a-Lactalbumin"... 

A), (B), (C), and (D), a Helix A, B, C, and D. DHEL Domestic hen egg-white lysozyme a-LA, baboon a-lactalbumin. Numbers in parentheses signify equivalent residues in domestic hen egg-white lysozyme. Based on results by Acharya et al. (1989) and by Blake et al. (1967a). 

FiG. 8. Stereo view of the Ca(II) binding site in baboon a-lactalbumin. (Reproduced with permission from Phillips etal, 1987.)... 

We will see in Sections VII and X that the recent elucidation by X-ray crystallography of the binding sites for Ca(II) in baboon a-lactalbumin has led to a flurry of studies of potential binding by variants of lysozyme in a wide range of species. 

The studies in solution, by several schools, of the binding of calcium to a-lactalbumin, as well as the realization that crystals of a-lactalbumin (as ordinarily prepared) contain calcium, were of great importance in the elucidation of the three-dimensional structure of baboon a-lactalbumin by Phillips and co-workers, as already indicated. 

Following the establishment of a-lactalbumin as a Ca(II)-binding protein and the revision of the sequences of several a-lactalbumins (see above), Shewale et al. (1984) suggested that residues 82, 83, 87, and 88 were probably ligands for the calcium. In fact, the X-ray crystallographic studies of Phillips group showed that three of the predicted residues (82, 87, and 88) were involved. The Ca(II) proved to be seven coordinates in baboon a-lactalbumin In addition to the involvement of the residues shown in Table IX (see also Fig. 8), two water molecules are coordinated. The majority of the c-type lysozymes so far sequenced do not have the residues necessary for the coordination of Ca(II) (Table... 

IX). However, the establishment of the sequence of equine lysozyme, and soon afterward pigeon and echidna lysozymes, indicated the possibility that these three lysozymes bind Ca(II). This was confirmed for the equine and echidna proteins in Canberra, and for the equine and pigeon proteins in Sapporo. However, the echidna lysozyme sequence is the only one in which all residues are identical with the equivalent residues in baboon a-lactalbumin. This matter is discussed further in Section X. 

B. Robson, and E. Plan, J. Comput.-Aided Mol. Design, 4, 369 (1990). Comparison of the X-Ray Structure of Baboon a-Lactalbumin and the Tertiary Prediaed Computer Models of Human a-Laaalbumin. 

Two variants of a-lactalbumin, caprine and ovine, have no Met residues, indicating that this residue plays no direct role in the lactose synthase system. Of the lysozymes only baboon milk and pigeon egg-white lysozymes have no Met residues. 

The X-ray crystallographic determination of the structure of a-lactalbumin has been made on baboon (Papio cynocephalus) a-lactalbu-min. Its amino acid sequence has not been determined. However, on the basis of preliminary work by R. Greenberg (personal communication), it is evident, as would be expected, that its sequence is similar to that of human a-lactalbumin. On the basis of their X-ray studies, Acharya et al. (1989) concluded that the following are possible sequence changes from that of the human protein ... 

The chain lengths of all but three of the a-lactalbumins, considered in Section B, are 123 residues. One, rat a-lactalbumin, has a chain extension of 17 residues, giving 140 residues total. In their nucleotide sequence study Qasba and Safaya (1984) concluded that this extension arises from a T-to-G base change in the termination codon. Two sequences have fewer than 123 residues Rabbit has 122 residues and red-necked wallaby has 121 residues. The majority of the mammalian lysozymes (human, baboon, and equine milk rat urine and camel, pig 3, and langur stomach) have 130 residues. Bovine, caprine, and deer stomach lysozymes have 129 residues. Although pig stomach lysozyme 3 has 130 residues, two of its variants (1 and 2) have 128... 

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