Aspergillus fumigatus Chitinase A1

Adama Berndt '17

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Contents:

I. Introduction
II. General Structure of Chitinases
III. Chitinase Active Site
IV. Acetazolamide Binding
V. Concluding Thoughts
VI. References

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I. Introduction

Aspergillus fumigatus is a highly common airborne species of mold and the primary cause of the disease aspergillosis - characterized by allergenic symptoms. Traditionally A. fumigatus is considered to be an opportunistic pathogen, predominantly affecting immunocompromised individuals, and as the numbers of immunosuppressed individuals arise, unfortunately so too does the prevalence of aspergillosis(Latge, 2009). When thinking of targets for anti-fungal activity, the cell wall of microbial organisms and related processes has proved fruitful, as in the case of Beta-lactam antibiotics(Kong et.al, 2010).

However, microbial fungi have their own unique cell wall target in Beta(1,4)-linked N-acetylglucosamine (GlcNAc) polymer - Chitin. That’s why it is valuable to look at enzymes such as Chitinase A1(ChiA), which breaks the glycosidc bonds in Chitin. It may seem counter-intuitive to inhibit an enzyme that breaks down cell walls in fungi, but because ChiA’s function is necessary for A. fumigatus sexual reproduction, inhibiting it rather than promoting its natural function can disrupt and hopefully halt growth of A. fumigatus and potentially alleviate symptoms of aspergillosis. One study screened for inhibitors of plant chitinase’s and settled on acetazolamide to analyze the properties of Chitinase, when acetazolamide is bound.. 

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II. General Structure of Chitinases

The AA sequence of Chitinases ultimately dictates an eight distinct stranded alternating alpha/beta barrel structure also known as a Tim Barrel . The TIM barrel is shaped in a toroid fashion. The TIM barrel toroid can also be thought of more simply as a doughnut-like shape, with the alpha-helices forming the outer surface, and the alternating beta-sheets forming the inner surface of the toroid. Each of the eight primary alpha-helices is connected on either end in a beta- alpha-beta motif . 

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III. Chitinase Active Site

The active site of AfChiA is characterized by residues Trp312, Tyr34, Gln230, Met310, Ala205, Tyr232 and "Asp172 . The carbonyl group of Asp172 engages in hydrogen bonding with the amide group and Tyr232 with the amino acid’s carbonyl oxygen. Under normal circumstances, it is primarily the Asp residue that interacts directly with a chitin molecule to catalyze separation of glycosidic linkages .

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IV. Acetazolamide Binding

The thiadiazole ring of the inhibitor engages in stacking interaction with Trp312. Nitrogen groups contained in thiadiazole rings primarily hydrogen bond with amides of residues Ala124 and through much farther water-mediated contact in the case of Tyr125 .

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V. Concluding Thoughts

Ultimately ChiA1 is only one of many different chitinases. In the 5 plant chitinases, acetazolamide showed similarly strong binding profiles, therefore similar and significant inhibibition of all other plant-type chitinases in A. fumigatus is likely. Further optimization of acetazolamide, via decrease in size of functional groups, would be the next best way to explore chitinase inhibition.

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VI. References

Branden, Carl-Ivar, and John Tooze. Introduction to Protein Structure. New York: Garland Pub., 1991. 47-50. Web.

Gros, Ludovic, David E. Blair, Julie A. Frearson, Daan M.f. Van Aalten, Ian H. Gilbert, and Alexander W. Schuttelkopf. "Acetazolamide-based Fungal Chitinase Inhibitors." Bioorganic and Medicinal Chemistry 18.23 (2010): 8334-340. Web. 3 Dec. 2016.

Kong, Kok-Fai, Lisa Schneper, and Kalai Mathee. "Beta-lactam Antibiotics: From Antibiosis to Resistance and Bacteriology." Apmis 118.1 (2010): 1-36. Web. 3 Dec. 2016.

Latge', Jean Paul. "Aspergillus Fumigatus and Aspergillosis." Clinical Microbiology Reviews(2009): 310-50. Web. 3 Dec. 2016.

Orikoshi, H., S. Nakayama, K. Miyamoto, C. Hanato, M. Yasuda, Y. Inamori, and H. Tsujibo. "Roles of Four Chitinases (ChiA, ChiB, ChiC, and ChiD) in the Chitin Degradation System of Marine Bacterium Alteromonas Sp. Strain O-7." Applied and Environmental Microbiology 71.4 (2005): 1811-815. Web. 3 Dec. 2016.

Robertus, Jon D., and Arthur F. Monzingo. "The Structure and Action of Chitinases." Chitin and Chitinases (1999): 125-35. Web. 3 Dec. 2016.

Ubhayasekera, Wimal. "Structure and Function of Chitinases from Glycoside Hydrolase Family 19." Polymer International 60.6 (2011): 890-96. Web. 3 Dec. 2016.

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