Insensitivity of homocitrate synthase in extracts of Penicillium chyrosogenum to feedback inhibition by lysine

Evaluation of lysine biosynthesis as an antifungal drug target: biochemical characterization of Aspergillus fumigatus homocitrate synthase and virulence studies

Aspergillus fumigatus is the main cause of severe invasive aspergillosis. To combat this life-threatening infection, only limited numbers of antifungals are available. The fungal alpha-aminoadipate pathway, which is essential for lysine biosynthesis, has been suggested as a potential antifungal drug target. Here we reanalyzed the role of this pathway for establishment of invasive aspergillosis in murine models. We selected the first pathway-specific enzyme, homocitrate synthase (HcsA), for biochemical characterization and for study of its role in virulence. A. fumigatus HcsA was specific for the substrates acetyl-coenzyme A (acetyl-CoA) and alpha-ketoglutarate, and its activity was independent of any metal ions. In contrast to the case for other homocitrate synthases, enzymatic activity was hardly affected by lysine and gene expression increased under conditions of lysine supplementation. An hcsA deletion mutant was lysine auxotrophic and unable to germinate on unhydrolyzed proteins given as a sole nutrient source. However, the addition of partially purified A. fumigatus proteases restored growth, confirming the importance of free lysine to complement auxotrophy. In contrast to lysine-auxotrophic mutants from other fungal species, the mutant grew on blood and serum, indicating the existence of high-affinity lysine uptake systems. In agreement, although the virulence of the mutant was strongly attenuated in murine models of bronchopulmonary aspergillosis, virulence was partially restored by lysine supplementation via the drinking water. Additionally, in contrast to the case for attenuated pulmonary infections, the mutant retained full virulence when injected intravenously. Therefore, we concluded that inhibition of fungal lysine biosynthesis, at least for disseminating invasive aspergillosis, does not appear to provide a suitable target for new antifungals.

Biosynthesis of methionine-derived glucosinolates in Arabidopsis thaliana: recombinant expression and characterization of methylthioalkylmalate synthase, the condensing enzyme of the chain-elongation cycle

The major class of glucosinolates in Arabidopsis thaliana (L.) Heynh. are biosynthesized from methionine involving a three-step chain-elongation cycle. Each passage through the cycle results in the net addition of a single methylene group, with up to six cycles of elongation occurring in A. thaliana. The first reaction of the cycle is catalyzed by a methylthioalkylmalate synthase (MAMS), which condenses a omega-methylthio-2-oxoalkanoic acid with acetyl-CoA. Here we have demonstrated that MAM1, one of two similar genes in the A. thaliana ecotype Columbia, encodes a MAMS catalyzing the condensing reactions of the first two elongation cycles but not those of further cycles. The Columbia ecotype is dominated by compounds that have undergone only two elongation cycles. The A. thaliana MAM1 protein exhibits basic sequence similarity to other previously described enzymes catalyzing the condensation of 2-oxo acids and acetyl-CoA, such as isopropylmalate synthase (EC 2.3.3.13), an enzyme of leucine biosynthesis, and homocitrate synthase (EC 2.3.3.14). It also shares similar properties with them, including the catalytic requirements for a divalent metal ion and an adenine nucleotide. However, the MAM1 protein does not show activity with the substrates of any of these other enzymes, and was chromatographically separable from isopropylmalate synthase in extracts of A. thaliana. Thus, MAM1 is exclusively an enzyme of secondary metabolism, distinct from primary metabolic enzymes catalyzing similar reactions.

Homocitrate synthase from Penicillium chrysogenum. Localization, purification of the cytosolic isoenzyme, and sensitivity to lysine

Subcellular fractionation of cell-free extracts obtained by nitrogen cavitation showed that Penicillium chrysogenum Q176 contains a cytosolic as well as a mitochondrial homocitrate synthase activity. The cytosolic isoenzyme was purified about 500-fold, and its kinetic and molecular properties were investigated. Native homocitrate synthase shows a molecular mass of 155 +/- 10 kDa as determined by gel filtration and a pH of 4.9 +/- 0.1 as determined by chromatofocusing. The kinetic behaviour towards 2-oxoglutarate is hyperbolic, with Km = 2.2 mM; with respect to acetyl-CoA the enzyme shows sigmoidal saturation kinetics, with [S]0.5 = 41 microM and h = 2.6. The enzyme was inhibited strongly by L-lysine (Ki = 8 +/- 2 microM; 50% inhibition by 53 microM at 6 mM-2-oxoglutarate), competitively with 2-oxoglutarate, in protamine sulphate-treated and desalted cell-free extracts and in partially purified preparations. The extent of this inhibition was strongly pH-dependent. Both isoenzymes are equally susceptible to inhibition by lysine. The same inhibition pattern is shown by the enzyme from strain D6/1014A, which is a better producer of penicillin than strain Q176.

Inhibition and repression of homocitrate synthase by lysine in Penicillium chrysogenum

Homocitrate synthase in the first enzyme of the lysine biosynthetic pathway. It is feedback regulated by L-lysine. Lysine decreases the biosynthesis of penicillin (determined by the incorporation of [14C]valine into penicillin) by inhibiting and repressing homocitrate synthase, thereby depriving the cell of alpha-aminoadipic acid, a precursor of penicillin. Lysine feedback inhibited in vivo the biosynthesis and excretion of homocitrate by a lysine auxotroph, L2, blocked in the lysine pathway after homocitrate. Neither penicillin nor 6-aminopenicillanic acid exerted any effect at the homocitrate synthase level. The molecular mechanism of lysine feedback regulation in Penicillium chrysogenum involved both inhibition of homocitrate synthase activity and repression of its synthesis. In vitro studies indicated that L-lysine feedback inhibits and represses homocitrate synthase both in low- and high-penicillin-producing strains. Inhibition of homocitrate synthase activity by lysine was observed in cells in which protein synthesis was arrested with cycloheximide. Maximum homocitrate synthase activity in cultures of P. chrysogenum AS-P-78 was found at 48 h, coinciding with the phase of high rate of penicillin biosynthesis.