The structure of intrinsic factor (IF) in complex with cobalamin (Cbl) was determined at 2.6-Å resolution. The overall fold of the molecule is that of an ␣6/␣6 barrel. It is a two-domain protein, and the Cbl is bound at the interface of the domains in a base-on conformation. Surprisingly, two full-length molecules, each comprising an ␣-and a ␤-domain and one Cbl, and two truncated molecules with only an ␣-domain are present in the same asymmetric unit. The environment around Cbl is dominated by uncharged residues, and the sixth coordinate position of Co 2؉ is empty. A detailed comparison between the IF-B12 complex and another Cbl transport protein complex, trans-Cbl-B12, has been made. The pH effect on the binding of Cbl analogues in transport proteins is analyzed. A possible basis for the lack of interchangeability of human and rat IF receptors is presented.5] is a water-soluble vitamin and is essential for the growth and development of all mammals, including humans. Cbl is a member of the corrinoid series of cobalt-containing compounds and is distinguished from some other members of this group by possessing a nucleotide side chain terminating in dimethylbenzimidazole (1, 2). It serves as a key enzymatic cofactor for a number of methyltransferase and mutase reactions occurring in nature (3). In mammals, it contributes the prosthetic group for two important enzymes, methionine synthase (a methyltransferase) and methylmalonylCoA mutase (1, 2, 4).In mammals, three proteins are involved in the uptake, transport, and storage of Cbl. These are gastric intrinsic factor (IF), trans-Cbl II (TC), and haptocorrin (HC). These are immunologically distinct proteins with a protein core of Ϸ46 kDa. IF and HC are heavily glycosylated, but TC is not (5, 6). In humans, the genes for IF and HC are located in chromosome 22 (7,8), whereas the gene for TC is located in chromosome 11 (9). Human IF contains 399-aa residues plus Ϸ15% carbohydrate, giving it a molecular mass of Ϸ60 kDa, as observed by gel filtration (10-13). All three proteins promote Cbl entry through endocytosis involving distinct cell surface receptors (5,(14)(15)(16)(17)(18)(19)(20).Dietary Cbl is bound first to HC in the gastric lumen but is transferred to IF in the duodenum after degradation of HC by pancreatic proteases. IF is responsible for transit of Cbl through the small intestine and delivery of it to the endothelial cells that line the ileum. The IF-Cbl complex is then recognized by the IF-Cbl receptor, cubilin (CUB), located on the luminal side of the intestinal mucosal cells, and mediates its internalization. Lysosomal degradation of the internalized IF-Cbl complex releases Cbl, which is then able to bind to TC, probably within the enterocyte. The TC-Cbl complex is then translocated across the basolateral membrane and released into the circulation, to be taken up by TC-Cbl receptor-mediated process by all cells in the body. Lysosomal dissociation of the complex then occurs along with transfer of Cbl to the coenzyme methyl-and Ado-Cbl in the cytoplasm and i...

The solution and solid structure of Boc-l-Val-ΔPhe-ΔPhe-ΔPhe-l-Val-OMe, containing three consecutive ΔPhe residues, have been determined by X-ray diffraction, nuclear magnetic resonance, and circular dichroism methods. The crystals grown from aqueous methanol are orthorhombic, space group P212121, a = 11.624(2), b = 17.248(2), c = 21.532 Å, V = 4216 (1) Å3, Z = 4. In the solid state, the peptide exhibits a left-handed 310-helical conformation, in spite of the presence of two l-Val residues. NMR and CD studies in different solvents also support the crystal structure data, suggesting that the solid state structure is maintained in solution as well. This is the first report of a dehydropeptide containing three consecutive ΔPhe residues and exhibiting left-handed 310-helical conformation, which demonstrates the remarkable conformational consequences produced by consecutive occurrence of ΔPhe residues in a peptide.

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The i + 5 + i hydrogen bonded turn conformation (a-turn) with the fifth residue adopting a L conformation is frequently found at the C-terminus of helices in proteins and hence is speculated to be a "helix termination signal.'' An analysis of the occurrence of i + 5 + i hydrogen bonded turn conformation at any general position in proteins (not specifically at the helix C-terminus), using coordinates of 228 protein crystal structures determined by X-ray crystallography to better than 2.5 A resolution is reported in this paper. Of 486 detected a-turn conformations, 367 have the (i + 4)th residue in a L conformation, generally occurring at the C-terminus of a-helices, consistent with previous observations. However, a significant number (1 11) of a-turn conformations occur with (i + 4)th residue in 01, conformation also, generally occurring in a-helices as distortions either at the terminii or at the middle, a novel finding. These two sets of a-turn conformations are referred to by the names aaL and aaRturns, respectively, depending upon whether the (i + 4)th residue adopts a,. or a , conformations. Four a-turns, named ab,-turns, were noticed to be mirror images of a,,-turns, and four more a-turns, which have the (i + 4)th residue in @ conformation and denoted as a0-turns, occur as a part of hairpin bend connecting twisted @-strands. Consecutive a-turns occur, but only with x,,-turns. The preference for amino acid residues is different in a , , and *,,-turns. However, both show a preference for Pro after the C-termini. Hydrophilic residues are preferred at positions i + 1, i + 2, and i + 3 of aaL-turns, whereas positions i and i + 5 prefer hydrophobic residues. Residue i + 4 in a,,~-turns is mainly Gly and less often Asn. Although aaR-turns generally occur as distortions in helices, their amino acid preference is different from that of helices. Poor helix formers, such as His, Tyr, and Asn, also were found to be preferred for a,,-turns, whereas good helix former Ala is not preferred. a-Turns in peptides provide a picture of the a-turn at atomic resolution. Only nine peptide-based a-turns are reported so far, and all of them belong to *,,-turn type with an achiral residue in position i + 4. The results are of importance for structure prediction, modeling, and de novo design of proteins.Keywords: database analysis; helix distortion; helix termination; hydrogen bond; loop structure a-Helices and @-sheets are the most common stabilizing structures in proteins. However, proteins cannot attain the globularity but for the so called random coils or loop regions that provide a directional change for the polypeptide chain and act as a linking motif between different secondary structures, such as a-helix and/or @-strands. Moreover, random coils are usually situated at the surface of proteins (Kuntz, 1972;Rose, 1978) and therefore serve as loci for molecular recognition, activity, and specificity (Rose et al., 1985). Hence, the loop regions are considered important for the structure and function of proteins. Although th...