Ascorbic crystal structure

Z = 8 Dx = 3.09 R = 0.076 for 1,253 intensities. The crystal structure contains two symmetry-independent molecules. The two l-ascorbate ions differ significantly only in the orientations of the terminal hydroxyl groups of the side chains, with C-6 - 0-6 to C-5 - 0-5 gauche and trans, respectively. The unprotonated oxygen-atoms are 0-1 and 0-3, with C-O distances ofbetween 127 and 133 pm. One Tl+atom has four oxygen atoms at distances of 258 to 298 pm the other has three, at distances of 258,268, and 300 pm. The 0-3 atoms have two Tl+ contacts,... 

The crystal structure of D-gulono-1,4-lactone (2) has been determined,42 and is closely approximated by formula 38. Interestingly, the conformation of the side chain in 38 differs from that in crystalline L-ascorbic acid,1 very probably because of different steric interactions between the 3- and 5-hydroxyl groups in 2 (that is, 38) and 6. Bridgman43 measured the compressibility of gulono-1,4-lactone crystals. 

The type 1-3 terminology to distinguish different Cu protein active sites remains extremely useful. Sub-groupings are appearing however in all three categories particularly in the case of the binuclear (EPR inactive) type 3 centers. Thus, in the recently determined X-ray crystal structure of ascorbate oxidase the type 3 and type 2 centers are present as a single trimer unit [. A discrete binuclear type 3 center is, however, retained in hemocyanin [6]. 

While there is at present no full understanding as to why plastocyanin should require two sites for reaction, there is now much evidence detailing this two-site reactivity. Moreover, the recent X-ray crystal structure of ascorbate oxidase (which has 4 Cu atoms per molecule) has indicated a plastocyanin-like domain, with the two type 3 Cu s (in close proximity with the type 2 Cu) located at the remote site. Fig. 2 [5]. Since electrons are transferred, from the type 1 Cu to O2 bound at the type 3 center this structure defines two very similar through-bond routes for biological electron transfer. 

The recent X-ray crystal structure of ascorbate oxidase [6] has indicated the relative positions of type 1, 2 and 3 Cu centers. The type 1 center is in a plastocyanin like domain, and is the primary acceptor of electrons from substrate. The shortest pathway for electron transfer from the type 1 to type 3 Cu s is the bifurcated path via Cys508 and either His 507 or His509. The two histidines are part of the plastocyanin-like domain, and serve also to coordinate the type 3 Cu s, Fig. 2. The His507 to Cys508 bonding is similar to that of Tyr83... 

Ascorbic acid ( Vitamin C ). The crystal structure of this substance cannot be said to be established with the certainty and precision we associate with those already described nevertheless, there is no reason to doubt that the structure suggested by Cox and Goodwin (1936) on the basis of p, limited study of the X-ray reflections is essentially correct. The work is described here because this crystal structure presents some very interesting and instructive features. It is also historically interesting because a preliminary study by optical and X-ray methods played a part in the elucidation of the chemical structure of this biologically important substance. (Cox, 1932 a Cox, Hirst, and Reynolds, 1932 Cox and Hirst, 1933.)... 

The oxidative mechanism of RNR R2 differs from that of MMOH in requiring an additional electron, since Tyrl22 provides only a single electron. This electron is needed to convert P to X. It has been shown that external reductants such as excess Fe(II) or ascorbate can provide this electron in in vitro reconstitution reactions [87,97], Since the diiron site is buried 10 A below the protein surface, a long-range electron transfer pathway is required to deliver this extra electron to the diiron center. Such a pathway involving a number of amino acid residues has been proposed from examining the crystal structure of R2 [98],... 

The second route to an ascorbic acid analog consisted in oxidation by bromine to a 2-ketoacid with the same number of carbon atoms, enoliza-tion, and lactonization. The product was not crystallized, but the optical rotation of the sirup, [ ]d —20° (in water), was in fair agreement with that for the expected D-arabo-ascorbic acid. The sirup was strongly reducing and gave the absorption peak at 2450 A. characteristic of the ascorbic-acid structure. 

The crystal structure of the resting form of ascorbate oxidase from zucchini has recently been refined to 1.9 A resolution (Messerschmidt et al., 1992) the subunits of 552 residues (70000 Mr) are arranged as tetramers with D2 symmetry. Asp 92 is the attachment site for one of the two N-linked sugar moieties, which has the defined elec-... 

Sharp KH, Mewies M, Moody PC et al (2003) Crystal structure of the ascorbate peroxidase-ascorbate complex. Nat Struct Biol 10 303-307... 

The low-temperature MCD and absorption titration studies (Figure 10) have determined that azide binds to both the type 2 and type 3 centers with similar binding constants. A series of chemical perturbations and stoichiometry studies have shown that these effects are associated with the same azide. This demonstrates that one N3 bridges between the type 2 and type 3 centers in laccase. These and other results from MCD spectroscopy first defined the presence of a trinuclear copper cluster active site in biology (89). At higher azide concentration, a second azide binds to the trinuclear site in laccase. Messerschmidt et al. have determined from X-ray crystallography that a trinuclear copper cluster site is also present in ascorbate oxidase (87, 92) and have obtained a crystal structure for a two-azide-bound derivative (87). It appears that some differences exist between the two-azide-bound laccase and ascorbate oxidase derivatives, and it will be important to spectroscopically correlate between these sites. 

Patterson, W. R., and Poulos, T. L., 1995, Crystal structure of recombinant pea cytosolic ascorbate peroxidase, Biochemistry 34 4331n4341. 

The primary function of APX is to rid plant cells of toxic peroxides by using ascorbate as the reducing substrate. Pea cytosolic APX is the most thoroughly studied APX and was the first APX crystal stmcture determined." This now has been followed by the crystal structures of soybean cytosohc and tobacco stromal APX. The overall fold of APX is very similar to CcP except that APX is smaller and contains 250 residues compared to 294 in CcP. The main difference is that APX lacks the /3-sheet structure in CcP. APX also exists as a homodimer. 

The molecular structure of AA provides a few preferred bidentate metal-binding sites, which have been demonstrated in a few crystal structures of metal-ascorbate complexes. 

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