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Photosynthesis Research

, Volume 90, Issue 2, pp 125–148

First Online: 08 December 2006Received: 19 July 2006Accepted: 17 October 2006

Abstract

C-Labelled octulose phosphates were formed during photosynthetic CO2 fixation and were measured in spinach leaves and chloroplasts. Because mono- and bisphosphates of d-glycero-d-ido-octulose are the active 8-carbon ketosugar intermediates of the L-type pentose pathway, it was proposed that they may also be reactants in a modified Calvin–Benson–Bassham pathway reaction scheme. This investigation therefore initially focussed only on the ido-epimer of the octulose phosphates even though C-labelled d-glycero-d-altro-octulose mono- and bisphosphates were also identified in chloroplasts and leaves. CO2 predominantly labelled positions 5 and 6 of d-glycero-d-ido-octulose 1,8-P2 consistent with labelling predictions of the modified scheme. The kinetics of CO2 incorporation into ido-octulose was similar to its incorporation into some traditional intermediates of the path of carbon, while subsequent exposure to CO2 rapidly displaced the C isotope label from octulose with the same kinetics of label loss as some of the confirmed Calvin pathway intermediates. This is consistent with octulose phosphates having the role of cyclic intermediates rather than synthesized storage products. Storage products don’t rapidly exchange isotopically labelled carbons with unlabelled CO2.

A spinach chloroplast extract, designated stromal enzyme preparation SEP, catalysed and was used to measure rates of CO2 assimilation with Calvin cycle intermediates and octulose and arabinose phosphates. Only pentose but not arabinose phosphates and sedoheptulose 7-phosphate supported CO2 fixation at rates in excess of 120 μmol h mg Chl. Rates for octulose, sedoheptulose and fructose bisphosphates, octulose, hexose and triose monophosphates were all notably less than the above rate and arabinose 5-phosphate was inactive. Altro-octulose phosphates were more active than phosphate esters of the ido-epimer. The modified scheme proposed a specific phosphotransferase and SEP unequivocally catalysed reversible phosphate transfer between sedoheptulose bisphosphate and d-glycero-d-ido-octulose 8-phosphate. It was also initially hypothesized that arabinose 5-phosphate, an L-Type pentose pathway reactant, may have a role in a modified Calvin pathway. Arabinose 5-phosphate is present in spinach chloroplasts and leaves. Radiochromatography showed that C-arabinose 5-phosphate with SEP, but only in the presence of an excess of unlabelled ribose 5-phosphate, lightly labelled ribulose 5-phosphate and more heavily labelled hexose and sedoheptulose mono- and bisphosphates. However, failure to demonstrate any CO2 fixation by arabinose 5-phosphate as sole substrate suggested that the above labelling may have no metabolic significance. Despite this arabinose and ribose 5-phosphates are shown to exhibit active roles as enzyme co-factors in transaldolase and aldolase exchange reactions that catalyse the epimeric interconversions of the phosphate esters of ido- and altro-octulose. Arabinose 5-phosphate is presented as playing this role in a New Reaction Scheme for the path of carbon, where it is concluded that slow reacting ido-octulose 1,8 bisphosphate has no role. The more reactive altro-octulose phosphates, which are independent of the need for phosphotransferase processing, are presented as intermediates in the new scheme. Moreover, using the estimates of phosphotransferase activity with altro-octulose monophosphate as substrate allowed calculation of the contributions of the new scheme, that ranged from 11% based on the intact chloroplast carboxylation rate to 80% using the carboxylation rate required for the support of octulose phosphate synthesis and its role in the phosphotransferase reaction.

KeywordsArabinose 5- phosphate Calvin Cycle C-carbon dioxide Chloroplasts C-labelled novel compounds Keto-group exchange L-type pentose pathway Spinach leaf Octulose phosphates Octulose-P epimerization Phosphotransferase Revised RPP in PS Stromal enzymes AbbreviationsAldaldolase

Aldxaldolase exchange

Ara 5-Parabinose 5-phosphate

Ara 5-P Iarabinosephosphate isomerase

Chlchlorophyll

DHAPdihydroxyacetone phosphate

Ery 4-Perythrose 4-phosphate

FBP-asefructose 1,6-bisphosphatase

Fru 6-Pfructose 6-phosphate

Gapglyceraldehyde 3-phosphate

Gap-DHglyceraldehyde 3-phosphate dehydrogenase

Glc 6-Pglucose 6-phosphate

Glc 6-PDHglucose 6-phosphate dehydrogenase

Octoctulose

d-g-d-i-oct 1,8-P2d-glycero-d-ido-2-octulose 1,8-bisphosphate

d-g-d-a-oct 1,8-P2d-glycero-d-altro-2-octulose 1,8-bisphosphate

PBAphenylboronic acid

PFKphosphofructokinase

PGA3-phosphoglyceric acid

PGIphosphoglucoseisomerase

PPpentose phosphate pathway

PPEphosphoketopentose epimerase

PRIphosphoribose isomerase

PRKphosphoribulokinase

PSphotosynthesis

PTphosphotransferase: d-glycero-d-ido -and d-glycero-d-altro-octulose 1,8- bisphosphate :d-altro-heptulose 7-phosphotransferase

Rib 5-Pribose 5-phosphate

Ru 1,5-P2ribulose 1,5-bisphosphate

RPPreductive pentose pathway PS path of CO2 assimilation in C3 plants

Rubiscoribulose 1,5 bisphosphate carboxylase-oxygenase

Sehsedoheptulose altro-2-ketoheptulose

SBP-asesedoheptulose 1,7- bisphosphatase

SEPstromal enzyme preparation

TAtransaldolase

Taxtransaldolase exchange

TKtransketolase

TKxtransketolase exchange

TPItriosephosphate isomerase

ThPPthiamine pyrophosphate

Xlu 5-Pxylulose 5-phosphate

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Autor: John F. Williams - John K. MacLeod

Fuente: https://link.springer.com/







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