No abstract

Fungal microorganisms, including the human pathogenic yeast and filamentous fungi, are able to synthesize all proteinogenic amino acids, including nine that are essential for humans. A number of enzymes catalyzing particular steps of human-essential amino acid biosynthesis are fungi specific. Numerous studies have shown that auxotrophic mutants of human pathogenic fungi impaired in biosynthesis of particular amino acids exhibit growth defect or at least reduced virulence under in vivo conditions. Several chemical compounds inhibiting activity of one of these enzymes exhibit good antifungal in vitro activity in minimal growth media, which is not always confirmed under in vivo conditions. This article provides a comprehensive overview of the present knowledge on pathways of amino acids biosynthesis in fungi, with a special emphasis put on enzymes catalyzing particular steps of these pathways as potential targets for antifungal chemotherapy.

Functional and structural properties of several truncated or mutated variants of Candida albicans Gfa1p (glucosamine-6-phosphate synthase) were compared with those of the wild-type enzyme. Fragments encompassing residues 1-345 and 346-712 of Gfa1p, expressed heterogeneously in bacterial host as His6 fusions, were identified as the functional GAH (glutamine amidehydrolysing) and ISOM (hexose phosphate-isomerizing) domains respectively. It was found that the native GAH domain is monomeric, whereas the native ISOM domain forms tetramers, as does the whole enzyme. Spectrofluorimetric and kinetic studies of the isolated domains, the Delta218-283Gfa1p mutein and the wild-type enzyme revealed that the binding site for the feedback inhibitor, uridine 5'-diphospho-N-acetyl-D-glucosamine, is located in the ISOM domain. Inhibitor binding affects amidohydrolysing activity of the GAH domain and, as a consequence, the GlcN-6-P (D-glucosamine-6-phosphate)-synthetic activity of the whole enzyme. The fragment containing residues 218-283 is neither involved in ligand binding nor in protein oligomerization. Comparison of the catalytic activities of Gfa1p(V711F), Delta709-712Gfa1p, Gfa1p(W97F) and Gfa1p(W97G) with those of the native Gfa1p and the isolated domains provided evidence for an intramolecular channel connecting the GAH and ISOM domains of Gfa1p. The channel becomes leaky upon deletion of amino acids 709-712 and in the W97F and W97G mutants. The Trp97 residue was found to function as a molecular gate, opening and closing the channel. The W97G and V711F mutations resulted in an almost complete elimination of the GlcN-6-P-synthetic activity, with the retention of the amidohydrolase and sugar phosphate-isomerizing activities.

Two genes, LYS21 and LYS22, encoding isoforms of homocitrate synthase, an enzyme catalysing the first committed step in the lysine biosynthetic pathway, were disrupted in Candida albicans using the SAT1 flipper strategy. The double null lys21Δ/lys22Δ mutant lacked homocitrate synthase activity and exhibited lysine auxotrophy in minimal media that could be fully rescued by the addition of 0.5-0.6 mM L: -lysine. On the other hand, its virulence in vivo in the model of disseminated murine candidiasis appeared identical to that of the mother, wild-type strain. These findings strongly question a possibility of exploitation of homocitrate synthase and possibly also other enzymes of the lysine biosynthetic pathway as targets in chemotherapy of disseminated fungal infections.

No abstract

Acridine cell-penetrating peptide conjugates are an extremely important family of compounds in antitumor chemotherapy. These conjugates are not so widely analysed in antimicrobial therapy, although bioactive peptides could be used as nanocarriers to smuggle antimicrobial compounds. An octaarginine conjugate of an imidazoacridinone derivative (Compound 1-R8) synthetized by us exhibited high antifungal activity against reference and fluconazole-resistant clinical strains (MICs ≤ 4 μg mL−1). Our results clearly demonstrate the qualitative difference in accumulation of the mother compound and Compound 1-R8 conjugate into fungal cells. Only the latter was transported and accumulated effectively. Microscopic and flow cytometry analysis provide some evidence that the killing activity of Compound 1-R8 may be associated with a change in the permeability of the fungal cell membrane. The conjugate exhibited low cytotoxicity against human embryonic kidney (HEK-293) and human liver (HEPG2) cancer cell lines. Nevertheless, the selectivity index value of the conjugate for human pathogenic strains remained favourable and no hemolytic activity was observed. The inhibitory effect of the analysed compound on yeast topoisomerase II activity suggested its molecular target. In summary, conjugation with R8 effectively increased imidazoacridinone derivative ability to enter the fungal cell and achieve a concentration inside the cell that resulted in a high antifungal effect.

The susceptibility to several oligopeptide and amino acid antifungals of a Saccharomyces cerevisiae strain carrying multiple deletions in yeast multidrug resistance genes was compared to transformants containing the CDR1, CDR2, or MDR1 genes that encode the major Candida albicans drug efflux pumps. Recombinant yeast strains overexpressing Cdr1p and Cdr2p showed enhanced susceptibilities to all tested oligopeptide antifungals. The enhanced susceptibilities of multidrug-resistant yeast strains to oligopeptide antifungals corresponded to higher rates of oligopeptide uptake. Yeast cells overexpressing Cdr1p or Cdr2p effluxed protons at higher rates than the reference cells lacking these ABC transporters. An increased plasma membrane electrochemical gradient caused by the functional overexpression of Cdr1p or Cdr2p appeared to increase cellular susceptibility to oligopeptide antifungals by stimulating their uptake via oligopeptide permeases.

The LYS12 gene from Candida albicans, coding for homoisocitrate dehydrogenase was cloned and expressed as a His-tagged protein in Escherichia coli. The purified gene product catalyzes the Mg(2+)- and K(+)-dependent oxidative decarboxylation of homoisocitrate to α-ketoadipate. The recombinant enzyme demonstrates strict specificity for homoisocitrate. SDS-PAGE of CaHIcDH revealed its molecular mass of 42.6 ± 1 kDa, whereas in size-exclusion chromatography, the enzyme eluted in a single peak corresponding to a molecular mass of 158 ± 3 kDa. Native electrophoresis showed that CaHIcDH may exist as a monomer and as a tetramer and the latter form is favored by homoisocitrate binding. CaHIcDH is an hysteretic enzyme. The K(M) values of the purified His-tagged enzyme for NAD(+) and homoisocitrate were 1.09 mM and 73.7 μM, respectively, and k(cat) was 0.38 s(-1). Kinetic parameters determined for the wild-type CaHIcDH were very similar. The enzyme activity was inhibited by (2R,3S)-3-(p-carboxybenzyl)malate (CBMA), with IC(50) = 3.78 mM. CBMA demonstrated some moderate antifungal activity in minimal media that could be enhanced upon conversion of the enzyme inhibitor into its trimethyl ester derivative (TMCBMA). TMCBMA is the first reported antifungal for which an enzyme of the AAP was identified as a molecular target.