FIGURE 1 

The central enzymes of the Asp-derived amino-acid pathway in chloroplasts and allosteric regulation. (A) The amino acids Lys, Met, Thr and Ile, and the methylating agent AdoMet are synthesized from aspartate. Simple and double arrows indicate reactions treated as irreversible and reversible, respectively. Enzyme names are indicated in italics. (B) Regulatory map of the Asp-derived amino-acid pathway in Arabidopsis leaf mesophyll cell. The 13 enzymes constituting the core of the system are co-expressed in Arabidopsis mesophyll cell chloroplasts. Isoforms exist for monofunctional AKs (AK1, AK2), bifunctional AKs (AKI–HSDH I and AKII–HSDH II) and DHDPS (DHDPS1 and 2). AK3 and TS2 are not expressed in mesophyll cell chloroplasts (see Supplementary data). Genes for ASADH, HSK, CGS and allosteric TD are present as single copies in the Arabidopsis genome. Enzymes shown against a white background are non-allosteric, and those with a yellow background are allosteric. Bifunctional AK–HSDH proteins are symbolized with a linker between the two domains. Continuous lines symbolize interactions of high apparent affinity, and broken lines symbolize interactions of low apparent affinity. Boxed metabolites appear explicitly in the enzyme equation models. Other metabolite concentrations were taken into account but are subsumed in the apparent rate constants (see Supplementary data).