Dermatophyte lipids

Cryptic phosphorylation in nucleoside natural product biosynthesis

Kinases are annotated in many nucleoside biosynthetic gene clusters (BGCs) but generally are considered responsible only for self-resistance. Here, we report an unexpected 2’-phosphorylation of nucleoside biosynthetic intermediates in the nikkomycin and polyoxin pathways. This phosphorylation is a unique cryptic modification as it is introduced in the third of seven steps during aminohexuronic acid (AHA) nucleoside biosynthesis, retained throughout the pathway’s duration, and is removed in the last step of the pathway. Bioinformatic analysis of reported nucleoside BGCs suggests the presence of cryptic phosphorylation in other pathways and the importance of functional characterization of kinases in nucleoside biosynthetic pathways in general. This study also functionally characterized all of the enzymes responsible for AHA biosynthesis and revealed that AHA is constructed via a unique oxidative C-C bond cleavage reaction. The results suggest a divergent biosynthetic mechanism for three classes of antifungal nucleoside natural products.

Dermatophyte lipids-Composition and regulation of phospholipids

This review presents the extensive work carried out on lipid components of dermatophytes, their biosynthesis, turnover and regulation. It emerges from the work done so far that the pathways of lipid biosynthesis/ degradation and the lipid composition in dermatophytes are similar to those in yeasts and other fungi. Second messengers (Ca(2+), cAMP) were demonstrated to have a regulatory role in phospholipid metabolism and they mainly act by stimulating Ca(2+)/CaM or cAMP dependent protein kinase(s). Both these kinases were purified and characterized inMicrosporum gypseum. Further work is being carried out to elucidate the molecular mechanism of regulation of phospholipid metabolism by these second messengers.

Purification and characterization of cAMP dependent protein kinase from Microsporum gypseum

A cyclic AMP dependent protein kinase (PKA), its regulatory (R) and catalytic (C) subunits were purified to homogeneity from soluble extract of Microsporum gypseum. Purified enzyme showed a final specific activity of 277.9 nmol phosphate transferred min(-1) mg protein(-1) with kemptide as substrate. The enzyme preparation showed two bands with molecular masses of 76 kDa and 45 kDa on sodium dodecyl polyacrylamide gel electrophoresis. The 76 kDa subunit was found to be the regulatory (R) subunit of PKA holoenzyme as determined by its immunoreactivity and the isoelectric point of this subunit was 3.98. The 45 kDa subunit was found to be the catalytic (C) subunit by its immunoreactivity and phosphotransferase activity. Gel filtration using Sepharose CL-6B revealed the molecular mass of PKA holoenzyme to be 240 kDa, compatible with its tetrameric structure, consisting of two regulatory subunits (76 kDa) and two catalytic subunits (45 kDa). The specificity of enzyme towards protein acceptors in decreasing order of phosphorylation was found to be kemptide, casein, syntide and histone IIs. Purified enzyme had apparent K(m) values of 71 microM and 25 microM for ATP and kemptide, respectively. Phosphorylation was strongly inhibited by mammalian PKA inhibitor (PKI) but not by inhibitors of other protein kinases. The PKA showed maximum activity at pH 7.0 and enzyme activity was inhibited in the presence of N-ethylmaleimide (NEM) which shows the involvement of sulfhydryl groups for the activity of PKA. PKA phosphorylated a number of endogenous proteins suggesting the multifunctional role of cAMP dependent protein kinase in M. gypseum. Further work is under progress to identify the natural substrates of this enzyme through which it may regulate the enzymes involved in phospholipid metabolism.

Fungitoxicity of fatty acids against dermatophytes

Ten saturated and three unsaturated fatty acids over a wide spectrum of their carbon chain length were tested for their fungitoxicity against five dermatophytes. The experiments showed that short chain saturated fatty acids (C 7:0-C 11:0) are more toxic to dermatophytes than long chain ones (> C 12:0). The antifungal activity decreases with increasing carbon chain length in the even-numbered carbon chain series. Odd-numbered carbon fatty acids show irregularities in these respects. Undecanoic acid (C 11:0) is outstandingly most toxic in the C 7:0-C 18:0 series. Polyunsaturated fatty acids are more toxic than their corresponding saturated acids. The per cent growth inhibition decreased from day 4 to day 12 at all concentrations suggesting the degradation and/or utilization of fatty acids by the fungi during the observation period. The results are discussed on the background of the biochemistry of fatty acids.

Effect of keratin on the lipid composition of Scopulariopsis brevicaulis (Bainier)

Lipid content and composition of Scopulariopsis brevicaulis cultured with or without its natural substrate keratin was investigated. Lipid content was found to be lower in the presence of keratin (maximum 12% against 19% in the absence of keratin), with differences in levels of linoleic acid (C18 = 2), palmitic (C16:0) and to a lesser extent palmitoleic (C16 = 1) and alpha-linolenic (C18 = 3) acids. The degree of unsaturation was correspondingly lower in the organisms cultured with keratin as substrate.

Lipids of dermatophytes II. Effect of growth condition on the lipid composition and membrane transport of Microsporum gypseum

Supplementation of glucose-containing medium with ethanol and replacement of glucose by glycerol in the Sabouraud's growth medium of Microsporum gypseum altered the levels of total phospholipids as well as their apolar and polar head groups. The levels of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) increased under these growth conditions; also, the ratio of unsaturated/saturated phospholipid fatty acids decreased on ethanol supplementation but increased in the presence of glycerol. Steady state accumulation of labelled amino acids (glycine, lysine and aspartic acid) was affected under these conditions.

Lipid metabolism in fungi

So far, reviews that have appeared on fungal lipids present data mainly on the lipid composition of these organisms and the influence of lipids on their physiology. These reviews provide little information about the enzymes of lipid metabolism in these organisms and it is assumed, by most workers, that lipid synthesis in all fungi takes place as in Saccharomyces cervesiae, the only fungus in which the complete pathways of phospholipid biosynthesis have been worked out. During the last few years, literature has accumulated on lipid metabolic enzymes of other fungi, as investigators became increasingly interested in this area of research. The present review, after an introduction, will be divided into different sections and each section will deal, comparatively, with various aspects of fungal lipid metabolism and physiology. This review will, therefore, bring out the differences or similarities of lipid metabolism in diverse fungal species.

Lipids of dermatophytes

This investigation deals with phosphatides and fatty acid content of Epidermophyton floccosum, Microsporum cookie and Trichophyton mentagrophytes during different phases of growth. Total phosphatide content of these dermatophytes decreased with age, which was reflected in constituent major phosphatides. The zwitterionic and anionic phospholipids tended to maintain a constant ratio. Short chain fatty acids increased significantly with age in E. floccosum whereas these fatty acids represented a minor fraction of the total fatty acids in M. cookie and T. mentagrophytes. The ratio of saturated to unsaturated fatty acids increased 4-fold during growth in E. floccosum, whereas this increase was marginal in M. cookie. This ratio decreased in T. mentagrophytes.