Chloroplast NADP-malate dehydrogenase: structural basis of light-dependent regulation of activity by thiol oxidation and reduction

摘要: Abstract Background: NADP-dependent malate dehydrogenase (EC 1.1.1.82) is a light-activated chloroplast enzyme that functions in the C 4 pathway of photosynthesis. The light regulation believed to be mediated vivo by thioredoxin-catalyzed reduction and re-oxidation cystine residues. rates reversible activation inactivation are strongly influenced coenzyme substrates seem ultimately determine steady-state extent . Results: X-ray structure inactive, oxidized was determined at 2.8 A resolution. core homologous AND-dependent dehydrogenases. Two surface-exposed thioredoxin-accessible disulfide bonds present, one N-terminal extension other C-terminal extension. peptide constrained its bond fold into active site over NADP + , hydrogen bonding catalytic His225 as well obstructing access acid substrate. loops flanking site, termed Arg 2 Trp loops, contain substrate binding residues prevented from closing Conclusions: explains role inhibiting activity. negative terminus will interact more with positively charged nicotinamide than NADPH, explaining why coenzyme-binding affinities differ so markedly those all α -hydroxy may also slow dissociation upon reduction, providing mechanism for inhibition but not NADPH.