All animals and plants dynamically attach and remove O-linked β-N-acetylglucosamine (O-GlcNAc) at serine and threonine residues on myriad nuclear and cytoplasmic proteins. O-GlcNAc cycling, which is tightly regulated by the concerted actions of two highly conserved enzymes, serves as a nutrient and stress sensor. On some proteins, O-GlcNAc competes directly with phosphate for serine/threonine residues. Glycosylation with O-GlcNAc modulates signalling, and influences protein expression, degradation and trafficking. Emerging data indicate that O-GlcNAc glycosylation has a role in the aetiology of diabetes and neurodegeneration.

Cycling of O-linked β- N -acetylglucosamine on nucleocytoplasmic proteins

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

The structure and synthesis of the fungal cell wall.

Chitin is one of the most important biopolymers in nature. It is mainly produced by fungi, arthropods and nematodes. In insects, it functions as scaffold material, supporting the cuticles of the epidermis and trachea as well as the peritrophic matrices lining the gut epithelium. Insect growth and morphogenesis are strictly dependent on the capability to remodel chitin-containing structures. For this purpose, insects repeatedly produce chitin synthases and chitinolytic enzymes in different tissues. Coordination of chitin synthesis and its degradation requires strict control of the participating enzymes during development. In this review, we will summarize recent advances in understanding chitin synthesis and its degradation in insects.

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Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases

Polyphenol oxidases in plants and fungi: going places? A review.

Melanins are dark-brown to black pigments found in animals, plants, and microorganisms. These pigments are not essential for growth and develop­ ment, but rather they enhance the survival and competitive abilities of species in certain environments. The association of melanins with immune responses has been noted for plants (13) and invertebrates (180, 227, 239, 250). Melanin biosynthesis and function have been studied most extensively in animals, primarily because of the association of melanins with skin disorders (25,53, 188) and malignant melanomas (119). Melanins in the eye and inner ear have also received considerable attention because of their interactions with drugs (84). The synthesis of animal melanin from tyrosine by the action of tyrosinase is proven (52, 175, 270). Tyrosinase, the key enzyme in animal melanin synthesis, also occurs and is best characterized in the common mushroom (Agaricus bisporus) and Neurospora crassa (136). Consequently, some researchers have been hasty to conclude that fungal melanins also are derived from tyrosine. The importance of melanins for survival and longevity of fungal propagules has been recognized for many years (126, 235). During the past ten years three additional discoveries have increased interest among plant pathologists in fungal melanins. First, unique pathways for melanin biosynthesis were discovered in both Ascomycotina (16) and Basidiomycotina (234). Second, investigators discovered a new class of fungicides that prevent direct penetra­ tion of plant tissue by inhibiting melanin biosynthesis in appressorial cells (221, 223). Third, phytotoxins were discovered that are shunt products of

Biosynthesis and Functions of Fungal Melanins

Using N-acetylglucosaminono-1,5-lactone (1) as the reference, the inhibitory activity of its (formal) derivatives N-acetylglucosaminono-1,5-lactone oxime (2) and N-acetylglucosaminono-1,5-lactone O-(phenylcarbamoyl)-oxime (3) was tested against beta-N-acetylglucosaminidase of different origins (animal, plant, fungus) Displaying inhibition constants of 045 microM and 062 microM, for the animal and plant enzyme, respectively, the simple oxime 2 was about equally potent as the parent lactone 1, and 50-400 times more efficient than two recently described new beta-N-acetylglucosaminidase inhibitors The (phenylcarbamoyl)oxime 3 performed even better, particularly with the fungal enzyme (Ki = 40 nM), ie was about 350 times more potent than the lactone In all cases competitive inhibition was observed with 4-nitrophenyl-beta-N-acetylglucosaminide as the substrate With Ki/Km ratios up to 3300 for 2 and 13,600 for 3, the mode of action of these novel inhibitors is probably that of transition state mimicry Suggestions are made for their use as a tool in biological research

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1991.tb15976.x

N-acetylglucosaminono-1,5-lactone oxime and the corresponding (phenylcarbamoyl)oxime. Novel and potent inhibitors of beta-N-acetylglucosaminidase.

Cell wall-associated enzymes in fungi.

Summary
A purified wall fraction was prepared from the mycelium of Agaricus bisporus. The isolated wall consisted of 43% (w/w) chitin, 14% KOH-soluble glucan, 27% β-glucan, 16% protein and 1–5% lipid. Traces of mannose and xylose were detected by gas chromatography. The architecture of the wall was investigated with sequential enzyme digestion and electron microscopy. The outer wall surface is covered by a mucilage that is removed by the isolation procedure. The wall fraction could be completely degraded by extraction in warm I M-KOH followed by digestion with a mixed β-glucanase and then chitinase. The outer layer of KOH-soluble glucan is amorphous and of variable thickness. Incubation with glucanase did not change the dimensions of the wall but was necessary before the inner wall was susceptible to attack by chitinase, indicating that β-glucan does not constitute a separate wall layer but is a matrix associated with the fibrillar chitin. The inner layer presents an even, compact, fibrillar side as the inside surface of the wall, but is looser and uneven outwardly where it interdigitates with the irregular inner surface of the KOH-soluble glucan. Silver hexamide staining showed cystine-containing protein throughout the wall.

/pdf/chemistry-and-architecture-of-the-mycelial-wall-of-agaricus-1uuhkefgyx.pdf

Chemistry and Architecture of the Mycelial Wall of Agaricus bisporus

β-N-Acetylhexosaminidase: A target for the design of antifungal agents

SUMMARY: Enzymological evidence has been sought for the purported involvement of chitinolysis in vegetative growth of filamentous fungi. A procedure has been developed for the production of fast growing and morphologically homogeneous exponential phase mycelium of the non-septate dimorphic zygomycete Mucor rouxii. A partially purified extract of this material has been subjected to gel-permeation chromatography and the chitinolytic activity of eluate fractions has been assessed using colloidal and nascent chitin and 3,4-dinitrophenyl tetra-N-acetylchitotetraoside [3,4-DNP-(GlcNAc)4] as substrates. Exponentially growing (td = 1·1 h) mycelium consisting of single short-branched hyphae contains at least seven chitinases. The two particulate ones have not been studied in detail. The soluble chitinases hydrolyse (pseudo)chito-oligomers by random cleavage of internal β-1,4-bonds (and not by processing) and have a minimum chain-length requirement of n = 4. They are clearly distinct from β-N-acetylglucosaminidase (β-GlcNAc'ase) with respect to their chromatographic behaviour, substrate chain-length specificity, inhibition by chitobionolactone oxime (K
1 = 175 μM), and non-inhibition by the specific β-GlcNAc'ase inhibitor N-acetylglucosaminono-1,5-lactone oxime. Their pH optima are similar (6·5-7·0), and all can hydrolyse 3,4-DNP-(GlcNAc)4 as well as nascent chitin. With respect to their charge, response to protease treatment, behaviour upon gel-permeation chromatography and ability to use colloidal chitin as a substrate, the soluble chitinases do, however, represent two distinct groups. Type A chitinases are acidic, display partial latency, show an unusual affinity to dextran gel and act weakly on colloidal chitin. Type B chitinases are basic (or neutral) and non-zymogenic, do not behave anomalously upon gel filtration and can degrade preformed chitin. An hypothesis is presented for the function of the complex chitinolytic system of the fungal hypha in branching and, possibly, also in apical growth.

Dora M. Rast

Papers

N-acetylglucosaminono-1,5-lactone oxime and the corresponding (phenylcarbamoyl)oxime. Novel and potent inhibitors of beta-N-acetylglucosaminidase.

Cell wall-associated enzymes in fungi.

Chemistry and Architecture of the Mycelial Wall of Agaricus bisporus

β-N-Acetylhexosaminidase: A target for the design of antifungal agents

A complex chitinolytic system in exponentially growing mycelium of Mucor rouxii: properties and function.