The glycolipid involved in chitin synthesis by zoospores ofBlastocladiella emersonii is a monoglucosyldiacylglycerol

Insect chitin synthases: a review

Chitin is the most widespread amino polysaccharide in nature. The annual global amount of chitin is believed to be only one order of magnitude less than that of cellulose. It is a linear polymer composed of N-acetylglucosamines that are joined in a reaction catalyzed by the membrane-integral enzyme chitin synthase, a member of the family 2 of glycosyltransferases. The polymerization requires UDP-N-acetylglucosamines as a substrate and divalent cations as co-factors. Chitin formation can be divided into three distinct steps. In the first step, the enzymes' catalytic domain facing the cytoplasmic site forms the polymer. The second step involves the translocation of the nascent polymer across the membrane and its release into the extracellular space. The third step completes the process as single polymers spontaneously assemble to form crystalline microfibrils. In subsequent reactions the microfibrils combine with other sugars, proteins, glycoproteins and proteoglycans to form fungal septa and cell walls as well as arthropod cuticles and peritrophic matrices, notably in crustaceans and insects. In spite of the good effort by a hardy few, our present knowledge of the structure, topology and catalytic mechanism of chitin synthases is rather limited. Gaps remain in understanding chitin synthase biosynthesis, enzyme trafficking, regulation of enzyme activity, translocation of chitin chains across cell membranes, fibrillogenesis and the interaction of microfibrils with other components of the extracellular matrix. However, cumulating genomic data on chitin synthase genes and new experimental approaches allow increasingly clearer views of chitin synthase function and its regulation, and consequently chitin biosynthesis. In the present review, I will summarize recent advances in elucidating the structure, regulation and function of insect chitin synthases as they relate to what is known about fungal chitin synthases and other glycosyltransferases.

Biochemistry of chitin synthase

This article compiles the papers dealing with the biochemistry of chitin synthase (CS) published during the last decade, provides up-to-date information on the state of knowledge and understanding of chitin synthesis in vitro, and points out some firmly entrenched ideas and tenets of CS biochemistry that have become of age without hardly ever having been critically re-evaluated. The subject is dealt with under the headings "Components of the CS reaction" (educt, cation requirement and intermediates; product), "Regulation of CS" (cooperativity and allostery; non-allosteric activation or priming of CS; latency), "Concerted action of CS and enzymes of chitinolysis", "Inhibition of CS", "Multiplicity of CSs", and "Structure of CS" (the putative UDPGlcNAc-binding domain of CS; identification of CS polypeptides; glycoconjugation). The prospects are outlined of obtaining a refined three-dimensional (3D) model of the catalytic site of CS for biotechnological applications.

Potassium- and calcium-induced alterations in lipid interactions of isolated plasma membranes from blastocladiella emersonii. Evidence for an adenosine 5'-triphosphate requirement

The physical-chemical properties of the isolated plasma membranes from zoospores of the chytridiomycete Blastocladiella emersonii were investigated, with electron spin resonance (ESR) spectroscopy, using the spin-label 5-nitroxystearate (5-NS). Both isolated plasma membranes and aqueous dispersion of the lipids extracted from the plasma membranes were spin-labeled and analyzed. Plots of the hyperfine splitting parameter (2T) vs. temperature indicated that the middle break point, TM, initially observed in experiments with spin-labeled zoospores in vivo [Leonards, K. S., & Haug, A. (1980) Biochim. Biophys. Acta 600, 805-816], was the result of a lipid-lipid interaction (glycolipid-glycolipid or glycolipid-neutral lipid) rather than a lipid-protein interaction. This interaction was markedly affected by Ca2+ ions, which interacted directly with the lipid components, increasing TM from 11 +/- 1 (Ca2+ removed by EDTA) to 21 +/- 1 degree C (10 mM Ca2+) in the lipid dispersions and from 12 +/- 1 to 23 +/- 1 degree C in the plasma membrane preparations. The initial ESR studies on spin-labeled zoospores in vivo had also demonstrated that the addition of K+ ions could reverse the Ca2+ ion effect, downshifting TM from 22 +/- 1 to 10 +/- 1 degree C. The addition of of K+ ions to the isolated plasma membrane had no affect on TM, indicating that K+ ions do not simply replace Ca2+ ions but exert their effect indirectly on the membrane. However, after the inclusion of ATP, K+ ions could reverse the Ca2+ ion effect. it was determined that the ATp generated an "energized membrane" state which permitted the K+ ions to reverse the Ca2+ effect. Since K+ ions have been shown to depolarize the membrane potential in both zoospores and isolated zoospore plasma membrane preparations (generated by ATP), were suggest that the K+ ion induced reversal of the Ca2+ ion effect, and therefore the change in the lipid-lipid interactions responsible for TM, is a consequence of the K+ ion induced depolarization of the membrane potential.