Synthesis of different nucleoside 5′‐diphospho‐sulfoquinovoses and their use for studies on sulfolipid biosynthesis in chloroplasts

Abstract

6-Su~fo-a-~-quinovopyranosyl phosphate was reacted with different nucleoside monophosphate morpholidates to form ADP-, CDP-, GDP-and UDP-sulfoquinovose. Analytical and preparative HPLC of these nucleotides was performed on reversed-phase columns using volatile buffer systems as eluant. The isolated compounds were characterized by NMR spectroscopy (except the CDP derivative) and used for an investigation of sulfolipid biosynthesis by chloroplasts. For this purpose intact spinach chloroplasts were biosynthetically preloaded with radioactive diacylglycerol to provide a sulfoquinovosyl acceptor. When sulfosugar nucleotides were added to such prelabelled intact organelles, the background levels of sulfolipid biosynthesis did not rise. On the other hand, after osmotic shock of prelabelled chloroplasts sulfolipid labelling was significantly increased by the addition of UDP-or GDP-sulfoquinovose. The same stimulation was observed with isolated envelope membranes, and UDP-sulfoquinovose proved to be twice as active as the GDP derivative. From these results it was concluded that the final step in sulfolipid biosynthesis is catalyzed by a UDP-sulfoquinovose: 1,2-diacylglycero13-0-a-D-sulfoquinovosyltransferase. This chloroplast enzyme cannot use exogenously supplied sulfosugar nucleotides, which as membrane-impermeable compounds are expected to be formed in viwo within chloroplasts.The plant sulfolipid 1 ',2'-di-0-acyl-3'-0-(6-deoxy-6-sulfoa-D-glucopyranosy1)-sn-glycerol [I] is a membrane lipid found in higher plants, cyanobacteria and some other photosynthetic procaryotes [2]. With regard to the sulfur budget of plants, the sulfolipid represents a major component which accounts for about half of the organic sulfur in leaves [3]. In higher plants this unique glycolipid is confined to plastids [4]. In these organelles it is located in the two surrounding envelopes as well as in the inner thylakoid membranes [5], where part of it may be firmly bound to specific protein complexes [6 -81.After application of radioactive sulfate to plants, the sulfolipid is rapidly labelled [I, 2, 91. Despite this specific and easy labelling, several steps in sulfolipid biosynthesis have not been resolved and at present two alternative routes are discussed. It has been suggested that a sulfoglycolytic sequencc [lo -121 produces 3-sulfo-~-lactaldehyde which instead of 3-phospho-~-glyceraldehyde can be used by aldolase [lo] to form 6-deoxy-6-sulfo-~-fructofuranosyl 1-phosphate. Subsequent hydrolysis and isomerisation to 6-sulfoquinovose (quinovose = 6-deoxy-~-glucose), rephosphorylation at C-I and activation by a corresponding pyrophosphorylase could result in a nucleoside diphospho-sulfoquinovose. Such a com-

Crystal structure of SQD1, an enzyme involved in the biosynthesis of the plant sulfolipid headgroup donor UDP-sulfoquinovose

“…The SQDB͞SQD1 proteins show modest sequence similarity to sugar-nucleotide enzymes (4), and this similarity has been recently exploited by Essigmann and coworkers (9) to predict the structure of the A. thaliana SQD1 protein. As they also demonstrated that SQD1 binds NAD ϩ and contains characteristic conserved Y-XXX-K and glycine-rich (G-XX-G-XX-G) sequence patterns, SQD1 appears to be a member of the shortchain dehydrogenase͞reductase (SDR) family (10-12).The sugar-nucleotide UDP-sulfoquinovose is thought to be the headgroup donor for SQDG biosynthesis (1), a conclusion supported by in situ labeling experiments with isolated chloroplasts and using synthetic UDP-sulfoquinovose (13,14) and by the discovery of UDP-sulfoquinovose in bacterial sulfolipid mutants and other organisms (15, 16). Pugh et al (17) proposed a metabolic pathway for SQDG biosynthesis in which UDPglucose is first converted to a UDP-4-ketoglucose-5-ene intermediate, with subsequent addition of sulfite by some unknown donor.…”

“…The sugar-nucleotide UDP-sulfoquinovose is thought to be the headgroup donor for SQDG biosynthesis (1), a conclusion supported by in situ labeling experiments with isolated chloroplasts and using synthetic UDP-sulfoquinovose (13,14) and by the discovery of UDP-sulfoquinovose in bacterial sulfolipid mutants and other organisms (15, 16). Pugh et al (17) proposed a metabolic pathway for SQDG biosynthesis in which UDPglucose is first converted to a UDP-4-ketoglucose-5-ene intermediate, with subsequent addition of sulfite by some unknown donor.…”

Molecular aspects of plastid envelope biochemistry

“…Under these conditions, the steric placement of these enzymes within the membrane as well as their kinetic parameters may be relevant to the outcome of the competition. Heinz et al [63] synthesized different nucleoside 5'-diphospho-sulfoquinovoses and demonstrated that both UDP-and GDP-sulfoquinovose significantly increased sulfolipid synthesis by spinach chloroplasts and by isolated envelope membranes, UDP-sulfoquinovose being twice as active as the GDP derivative. Therefore, sulfolipid synthesis in envelope membranes (Fig.…”

Biosynthesis of Digalactosyldiacylglycerol in Plastids from 16:3 and 18:3 Plants

“…Purified chloroplasts were finally suspended in a buffer containing 0.33 to 0.37 M sorbitol (as indicated in Table I) and 25 mM Tricine (pH 7.6 with KOH). Spinach chloroplasts for acetate-labeling experiments were isolated by using a rapid purification procedure (19).…”