Penicillium sp. beta-Galactosidase

Morgan Stucke '26 and Sophia Percy '26

Contents:

I. Introduction

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In Penicillium sp., Beta-galactosidase cleaves the disaccharide lactose into the corresponding monosaccharides: beta-glucose and beta-galactose. This is a crucial step in breaking down lactose into other molecules that can be used for energy via other mechanisms. In Penicillium sp., beta-Galactosidase belongs to a family of 35 glycosyl hydrolases, which are proteins that break apart glycosidic linkages into the corresponding monosaccharides.  

II. General Structure

Beta-Galactosidase has a molecular weight of 520,000 daltons and is comprised of five distinct structural domains. The entire Psp-B-gal complex consists of 971 amino acid residues.  Domain I contains the catalytic site, the hydrolysis site (please see section III for more detail). It's comprised of beta/alpha barrels, and spans across 355 amino acid residues (Leu41-Gly395). Domain II encompasses Tyr396-Tyr576 and consists of 16 antiparallel beta sheets and an alpha helix at the C terminus. Domain III includes Trp577-Tyr665, with an alpha helix at the N terminus and 8 antiparallel beta sheets. There is a III/IV linker between domains III and IV. It spans across Thr666-Pro687, passing through Domain V. Domain IV spans across Glu688-Leu861 and is composed of 8 beta sheets arranged in a "jelly roll" motif. See another depiction of this motif.
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Figure 2: Cartoon depicting the layout of a "jelly roll" motif.

Domain V will encompass the rest of the protein, and is alligned in a "jelly roll" motif similar to that of Domain IV.

III. Hydrolysis Site

A single lactose molecule binds to Domain I, specifically amino acids Glu142, Ala141, Tyr95, Ile139, Asn199, Glu200, Glu299, Tyr261, Tyr343, and Tyr365 . Feel free to click on the protein and use the cursor to observe the amino acids at all angles. This is the catalytic site of beta-Galactosidase. Another layout of this catalytic site can be viewed below, with lactose represented as a blue form derived from electric field mapping. Hydrolysis of lactose takes place here via general acid catalysis. This requires a proton donor and a nucleophile, existing here as Glu 200 and Glu 299. These two aminio acids hydrolyze lactose into galactose and glucose, as mentioned in the introduction. Glu 200 is oriented correctly with respect to lactose by hydrogen-bonding with the side chain of Trp809, which sticks out of Domain IV.

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Figure 3: Visualization of lactose in the catalytic site.

IV. Applications

Beta-Galactosidase has many practical applications, some in the science world and others existing in aspects of everyday life. Beta-Galactosidases are used for the hydrolysis of lactose in dairy products, including milk and cheese, and for processing of other lactose-containing products. Thus, this protein attracts industrial attention for its enzymatic activity. Additionally, beta-Galactosidase is often used in colorimetric assays, a method that can identify the concentration of a certain chemical compound in a solution. The function of beta-Galactosidase is also quite simple, allowing it to be a good protein of study for new molecular biology students who seek to better understand protein function. It also participates in the lac operon, a common operon used as a baseline for understanding all other genetic operons, although it should be noted that the E. coli operon is frequently studied, not Penicillium sp’s.

V. References

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Davies, G., & Henrissat, B. (1995). Structures and mechanisms of glycosyl hydrolases. Structure (Vol. 3, Issue 9, pp. 853–859). Elsevier BV. https://doi.org/10.1016/s0969-2126(01)00220-9

Rojas, A. L., Nagem, R. A. P., Neustroev, K. N., Arand, M., Adamska, M., Eneyskaya, E. V., Kulminskaya, A. A., Garratt, R. C., Golubev, A. M., & Polikarpov, I. (2004). Crystal Structures of ꞵ-Galactosidase from Penicillum sp. and its Complex with Galactose. Journal of Molecular Biology, 343(5), 1281-1292. 

Stirk, H. J., Woolfson, D. N., Hutchinson, E. G., Thornton, J. M. (1992). Depicting topology and handedness in jellyroll structures. Federation of European Biochemical Societies, 308(1), 1-3.

Sturm, Noel. (2020). Regulation of Gene Expression, California State University. Retrieved from https://www2.csudh.edu/nsturm/CHEMXL153/RegulationofGeneExpression.htm.