Properties and mutation analysis of the CelK cellulose-binding domain from the Clostridium thermocellum cellulosome

被引:22
作者
Kataeva, IA
Seidel, RD
Li, XL
Ljungdahl, LG
机构
[1] Univ Georgia, Dept Biochem & Mol Biol, Athens, GA 30602 USA
[2] Univ Georgia, Ctr Biol Resources Recovery, Athens, GA 30602 USA
关键词
D O I
10.1128/JB.183.5.1552-1559.2001
中图分类号
Q93 [微生物学];
学科分类号
071005 ; 100705 ;
摘要
The family IV cellulose-binding domain of Clostridium thermocellum CelK (CBDCelK) was expressed in Escherichia coli and purified. It binds to acid-swollen cellulose (ASC) and bacterial microcrystalline cellulose (BMCC) with capacities of 16.03 and 3.95 mu mol/g of cellulose and relative affinities (K-r) of 2.33 and 9.87 liters/g, respectively. The CBDCelK is the first representative of family IV CBDs to exhibit an affinity for BMCC. The CBDCelK also binds to the soluble polysaccharides lichenin, glucomannan, and barley beta -glucan, which are substrates for CelK. It does not bind to xylan, galactomannan, and carboxymethyl cellulose. The CBDCelK contains 1 mol of calcium per mel. The CBDCelK has three thiol groups and one disulfide, reduction of which results in total loss of cellulose-binding ability. To reveal amino acid residues important for biological function of the domain and to investigate the role of calcium in the CBICelK, four highly conserved aromatic residues (Trp(56), Trp(94), Tyr(111), and Tyr(136)) and Asp(192) were mutated into alanines, giving the mutants W56A, W94A, Y111A, Y136A, and D192A. In addition 14 N-terminal amino acids were deleted, giving the CBD-N-CelK. The CBD-N-CelK and D192A retained binding parameters close to that of the intact CBDCelK, W56A and W94A totally lost the ability to bind to cellulose, Y136A bound to both ASC and BMCC but with significantly reduced binding capacity and K-r and Y111A bound weakly to ASC and did not bind to BMCC. Mutations of the aromatic residues in the CBDCelK led to structural changes revealed by studying solubility, circular-dichroism spectra, dimer formation, and aggregation. Calcium content was drastically decreased in D192A. The results suggest that Asp192 is in the calcium-binding site of the CBDCelK and that calcium does not affect binding to cellulose. The 14 amino acids from the N terminus of the CBDCelK are not important for binding. Tyr136, corresponding to Cellulomonas fimi CenC CBDN1 Y85, located near the binding cleft, might be involved in the formation of the binding surface, while Y111, W56A, and W94A are essential for the binding process by keeping the CBDCelK correctly folded.
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页码:1552 / 1559
页数:8
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