Triose structures

Triose structures. The two central structures for glyceraldehyde enantiomers are written according to the Mills convention (wedges out of the paper, dashed lines going in). The next structures are the full Fischer convention and the outer structures an abbreviated Fischer convention in which the hydrogen and central carbon atoms are implicit. 

Figure 2.20 Motifs that are adjacent in the amino acid sequence are also usually adjacent in the three-dimensional structure. Triose-phosphate isomerase is built up from four P-a-p-a motifs that are consecutive both in the amino acid sequence (a) and in the three-dimensional structure (b).

Another important parallel /3-array is the eight-stranded parallel j8-barrel, exemplified in the structures of triose phosphate isomerase and pyruvate kinase (Figure 6.30). Each /3-strand in the barrel is flanked by an antiparallel a-helix. The a-helices thus form a larger cylinder of parallel helices concentric with the /3-barrel. Both cylinders thus formed have a right-handed twist. Another parallel /3-structure consists of an internal twisted wall of parallel or mixed /3-sheet protected on both sides by helices or other substructures. This structure is called the doubly wound parallel j8-sbeet because the structure can be... 

The appearance of free iodine during the periodate oxidation of compounds having an active hydrogen atom (27) or an ene-diol structure (1,39) has frequently been observed, and this implies that further reduction of iodate, formed from periodate during the main reaction, takes place. It has, in fact, been shown that, in acid solution, iodate is fairly readily reduced by such compounds as triose reductone (27), dihydfoxy-fumaric (39), and tartronic (32) acids. 

The reaction of iodate with triose reductone is not only a function of the concentration of the reagents, it is also dependent on the pH of the solution. In solutions of triose reductone more dilute than 10"3M, iodine is set free from iodate, if the pH of the solution is lower than about 3 (55). Dihydroxyfumaric and L-ascorbic acids (26), which also have free ene-diol structures, behave similarly. 

Figure 5-6. Examples of tertiary structure of proteins. Top The enzyme triose phosphate isomerase. Note the elegant and symmetrical arrangement of alternating p sheets and a helices. (Courtesy of J Richardson.) Bottom Two-domain structure of the subunit of a homodimeric enzyme, a bacterial class II HMG-CoA reductase. As indicated by the numbered residues, the single polypeptide begins in the large domain, enters the small domain, and ends in the large domain. (Courtesy ofC Lawrence, V Rod well, and C Stauffacher, Purdue University.)...

N6. Neubauer, B. A., Pekrun, A., Eber, S. W Lakomek, M and Schroter, W., Relation between genetic defect, altered protein structure, and enzyme function in triose-phosphate isomerase (TPI) deficiency. Eur. J. Pediatr. 151,232a (1992). 

B-chains until they are acted on by R-enzyme, when maltose or malto-triose will be produced from the residual A-chain, and linear dextrins from the B-chains. The amount of maltose or maltotriose liberated on treating the /3-limit dextrin with R-enzyme will be a measure of the number of A-chains in the molecule, and from these data, the ratio of A B chains in the molecule can be calculated.220 Peat concluded that multiple branching is an intrinsic part of the amylopectin structure, as the observed yield of these sugars was greater than expected for a singly-branched structure. It should be noted that glycogen has been shown by similar enzymic methods to possess a truly random structure.221... 

Figure 4.9 (a) Triose phosphate isomerase (TIM), has a (3-a-(3 structure made up of eight P-a motifs terminating in a final a-helix, which form a barrel-like structure, (b) An open twisted P-sheet with helices on both sides, such as the coenzyme-binding domain of many dehydrogenases. (From Branden and Tooze, 1991. Reproduced by permission of Garland Publishing, Inc.)... 

Figure 3. Model of (p a)s TIM barrel from triose phosphate isomerase. Numbered arrow and twisted ribbon structures are beta sheets and alpha helicies, respectively. (Adapted and reproduced from Ref. 66 with permission. Cop3night 1984 American Association for the Advancement of Science.)...

Figure 2-28 The eight-fold oc/(3 barrel structure of triose phosphate isomerase. From Richardson. (A) Stereoscopic view. (B) Ribbon drawing. Courtesy of Jane Richardson.117...

Figure 13-6 Stereoscopic view into the active site of triose phosphate isomerase showing side chains of some charged residues PGH, a molecule of bound phosphoglycolohydroxamate, an analog of the substrate enolate.138 The peptide backbone, as an alpha-carbon plot, is shown in light lines.147 The (a/ (S)8-barrel structure is often called a TIM barrel because of its discovery in this enzyme. Courtesy of M. Karplus.

Two cell-wall fractions were obtained from mycelia of Neurospora crassa by differential extraction with alkali (soluble in 4%, and insoluble in 24%) on incubation with endo-(l - 3)-/3-D-glucanase, both gave473 D-glucose, laminara-biose and -triose, and 62-/3-D-glucosyllaminarabiose (118). exo-(1 - 3)-/3-D-Glucanase released D-glucose and gentiobiose. Structures were proposed for both fractions when this evidence and the results of methylation analysis were considered. 

Fig. 3 Mechanism of triose-P isomerase.4 Note the isoenergetic shift of the histidine between the two OH groups of the enediolate intermediate a low-barrier hydrogen bond is present in both structures.

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