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1. (Article ID: 6685)
 
Aleshin AE, Hoffmann C, Firsov LM, Honzatko RB
Refined crystal structures of glucoamylase from Aspergillus awamori var. X100
Journal of Molecular Biology 238 (1994) 575-591
 

The refined crystal structures of a proteolytic fragment of glucoamylase from Aspergillus awamori var. X100 have been determined at pH 6 and 4 to a resolution of 2.2 A and 2.4 A, respectively. The models include the equivalent of residues 1 to 471 of glucoamylase from Aspergillus niger and a complete interpretation of the solvent structure. The R-factors of the pH 6 and 4 structures are 0.14 and 0.12, respectively, with root-mean-square deviations of 0.014 A and 0.012 A from expected bondlengths. The enzyme has the general shape of a doughnut. The "hole" of the doughnut consists of a barrier of hydrophobic residues at the center, which separates two water-filled voids, one of which serves as the active site. Three clusters of water molecules extend laterally from the active site. One of the lateral clusters connects the deepest recess of the active site to the surface of the enzyme. The most significant difference in the pH 4 and 6 structures is the thermal parameter of water 500, the putative nucleophile in the hydrolysis of maltooligosaccharides. Water 500 is associated more tightly with the enzyme at pH 4 (the pH of optimum catalysis) than at pH 6. In contrast to water 500, Glu179, the putative catalytic acid of glucoamylase, retains the same conformation in both structures and is in an environment that would favor the ionized, rather than the acid form of the side-chain. Glycosyl chains of 5 and 8 sugar residues are linked to Asparagines 171 and 395, respectively. The conformations of the two glycosyl chains are similar, being superimposable on each other with a root-mean-square discrepancy of 1.9 A. The N-glycosyl chains hydrogen bond to the surface of the protein through their terminal sugars, but otherwise do not interact strongly with the enzyme. The structures have ten serine/threonine residues, to each of which is linked a single mannose sugar. The structure of the ten O-glycosylated residues taken together suggests a well-defined conformation for proteins that have extensive O-glycosylation of their polypeptide chain.

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2. (Article ID: 6768)
 
Aleshin A, Golubev A, Firsov LM, Honzatko RB
Crystal structure of glucoamylase from Aspergillus awamori var. X100 to 2.2-A resolution
Journal of Biological Chemistry 267 (1992) 19291-19298
 

The crystal structure of a catalytically active fragment of glucoamylase-I from Aspergillus awamori var. X100 has been determined to a resolution of 2.2 A. Twelve of its 13 alpha-helices are arranged into an "alpha/alpha-barrel." An inner core of six mutually parallel alpha-helices are connected to each other through a peripheral set of six alpha-helices. The peripheral helices are parallel to each other, but approximately antiparallel to the inner core of alpha-helices. The putative active site lies in the packing void of the inner set of helices. The last 30 residues of the enzyme comprise a separate domain containing 10 sites of O-glycosylation. Each instance of O-glycosylation involves a serine or threonine side chain linked to the alpha-anomer of a single mannosyl residue. The O-glycosylated domain is in an extended conformation, wrapping around the "waist" of the alpha/alpha-barrel. Two additional sites of N-glycosylation contribute well ordered glycosyl chains that lie in proximity to the belt of O-glycosylation. The model developed for glucoamylase is a rare and valuable structural example of a glycoprotein and an exo-acting amylolytic enzyme.

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