The stereochemistry of betulaprenol biosynthesis

By using stereospecifically double-labelled radioactive mevalonates it was shown that betulaprenols-6 to -9, found in the woody tissue of Betula verrucosa, each contain three biogenetically trans-isoprene residues and that the remaining residues are biogenetically cis. The results obtained with these radioactive mevalonates also indicated that the activity of isopentenyl pyrophosphate isomerase is low relative to the activity of prenyltransferase in this woody tissue. The incorporation of radioactive farnesyl pyrophosphate and radioactive geranylnerol and the lack of incorporation of radioactive geranylgeraniol into betulaprenols-6 to -9 demonstrated that they are formed by the cis-additions of isoprene residues to all-trans-farnesyl pyrophosphate.     

The structure and stereochemistry of artemin

Artemin was isolated from the aerial part of Artemisia maritima. Its structure and stereochemistry were determined on the basis of chemical transformations and spectral evidence     

The biological significance of methionine sulfoxide stereochemistry

Methionine can be oxidized by reactive oxygen species to a mixture of two diastereomers, methionine-S-sulfoxide and methionine-R-sulfoxide. Both free amino acid and protein-based forms of methionine-S-sulfoxide are stereospecifically reduced by MsrA, whereas the reduction of methionine-R-sulfoxide requires two enzymes, MsrB and fRMsr, which act on its protein-based and free amino acid forms, respectively. However, mammals lack fRMsr and are characterized by deficiency in the reduction of free methionine-R-sulfoxide. The biological significance of such biased reduction of methionine sulfoxide has not been fully explored. MsrA and MsrB activities decrease during aging, leading to accumulation of protein-based and free amino acid forms of methionine sulfoxide. Since methionine is an indispensible amino acid in human nutrition and a key metabolite in sulfur, methylation, and transsulfuration pathways, the consequences of accumulation of its oxidized forms require further studies. Finally, in addition to methionine, methylsulfinyl groups are present in various drugs and natural compounds, and their differential reduction by Msrs may have important therapeutic implications.     

The stereochemistry of cyclization in abscisic acid

The hydroxylation of the pro-6′-(R)-methyl of (+)-abscisic acid, which then cyclises to phaseic acid, was used to define the origin in mevalonate of the 6′-methyl groups. Abscisic acid (ABA), biosynthesised from [2-¹⁴C, 2-³H₂]-mevalonate, was metabolized to phaseic acid by tomato shoots. The slight loss of [³H] from the phaseate, and to a lesser extent from the ABA, suggested that the unlabelled 6′-methyl was hydroxylated. This was confirmed by Kuhn-Roth oxidation of methyl phaseate to give [¹⁴C, ³H]-acetate. The data also suggest that ABA is converted to dihydrophaseate via free phaseate, the conjugates being formed from each free acid.     

The constitution and absolute stereochemistry of persicaxanthin

Persicaxanthin, isolated from the fruits of peach, has been obtained in crystalline form. By spectroscopic (MS, ¹H NMR and CD) methods, its constitution and absolute stereochemistry have been determined as (3S,5R,6S)-5,6-epoxy-3-hydroxy-5,6-d The structural conclusions have received full support by comparison with semi-synthetic persicaxanthin obtained from natural violaxanthin.     

Dihydrofolate reductase. The stereochemistry of inhibitor selectivity

X-ray structural results are reported for 10 triazine and pyrimidine inhibitors of dihydrofolate reductase, each one studied as a ternary complex with NADPH and chicken dihydrofolate reductase. Analysis of these data and comparison with structural results from the preceding paper (Matthews, D.A., Bolin, J.T., Burridge, J.M., Filman, D.J., Volz, K.W., Kaufman, B. T., Beddell, C.R., Champness, J.N., Stammers, D.K., and Kraut, J. (1985) J. Biol. Chem. 260, 381-391) in which we contrasted binding of the antibiotic trimethoprim (TMP) to chicken dihydrofolate reductase on the one hand with its binding to Escherichia coli dihydrofolate reductase on the other, permit identification of differences that are important in accounting for TMP's selectivity. The crystallographic evidence strongly suggests that loss of a potential hydrogen bond between the 4-amino group of TMP and the backbone carbonyl of Val-115 when TMP binds to chicken dihydrofolate reductase but not when it binds to the E. coli reductase is the major factor responsible for this drug's more potent inhibition of bacterial dihydrofolate reductase. A key finding of the current study which is important in understanding why TMP binds differently to chicken and E. coli dihydrofolate reductases is that residues on opposite sides of the active-site cleft in chicken dihydrofolate reductase are about 1.5-2.0 A further apart than are structurally equivalent residues in the E. coli enzyme.     

The stereochemistry of hydrogen abstraction in vitamin K-dependent carboxylation

The stereochemistry of the hydrogen abstraction in the vitamin K-dependent carboxylation of synthetic peptides has been investigated; the carboxylation rates of various peptidic substrates containing a stereospecifically 4-monodeuterated glutamic acid residue have been compared to that of nondeuterated peptides. A significant isotope effect was found only with the substrates containing (4S)-4-deuterated glutamic acid. These data reveal that the rat liver microsomal vitamin K-dependent carboxylase acts stereospecifically in abstracting the 4-pro-S hydrogen of the glutamyl residue. The low values of the measured isotope effects indicate that the hydrogen abstraction does not constitute a limiting step in the carboxylation mechanism.     

The stereochemistry of beta-lactam formation in penicillin biosynthesis.

1. (2R,3S)-[U-14C,3-3H1]- and (2R,3R)-[U-14C,2,3-3H2] Cysteine hydrochlorides have been separately synthesised. The latter compound has been shown to have uniform distributions of tritium between C-2 and C-3. 2. The abvoe cysteines and (2R)-[U-14C,3,3,3',3'-3H4]cystine have been converted to samples of penicillin G by Penicillium chrysogenum. 3. Incorporation results indicate that all but 14% of the tritium is lost from the (2R,3S)-[3-3H1]isomer; that 42% of tritium is retained by the non-stereospecifically C-3 tritiated cystine; and that 58% of tritium is retained by the (2R,3R)-[2,3-3H2]isomer on conversion to penicillin G. 4. Degradation of the penicillin G derived from (2R,3R)-[U-14C,2,3-3H2]cysteine hydrochloride has indicated that in fact about 87% of the original C-3 tritium of cysteine is retained at C-5 of penicillin G. 5. The results indicate stereospecificity in the cyclisation giving rise to the beta-lactam ring in penicillin G in nature with loss of the 3-pro-S-hydrogen and rentention of the 3-pro-R-hydrogen of cysteine. Thus there is net retention of stereochemistry in the cyclisation.     

The stereochemistry of beta-lactam formation in cephalosporin biosynthesis.

3H and 14C from (2R,3S)[U-14C,3-3H1]cysteine and (2R,3R)-[U-14C,2,3-3H2]cysteine were incorporated into cephalosporin C by Cephalosporium acremonium. Analysis of the radioactive cephalosporin C indicated that the formation of its beta-lactam ring occurs stereospecifically and with retention of configuration at C-3 of cysteine.     

The absolute stereochemistry of a diterpene from Ballota aucheri

The semi-synthetic transformation of hispanolone, isolated from Ballota africana, into 6beta-hydroxy-15,16-epoxylabda-8,13(16),14-trien-7-one has established an ent-labdane absolute stereochemistry for a diterpene metabolite originally isolated from B. aucheri.