Thermodynamics of the Binding of the C-Terminal Repeat Domain of Streptococcus sobrinus Glucosyltransferase-I to Dextran
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- 作者：Hideyuki Komatsu ; Motoki Katayama ; Masaki Sawada ; Yukie Hirata ; Miyuki Mori ; Tetsuyoshi Inoue ; Kazuhiro Fukui ; Harumi Fukada ; Takao Kodama
- 刊名：Biochemistry
- 出版年：2007
- 出版时间：July 17, 2007
- 年：2007
- 卷：46
- 期：28
- 页码：8436 - 8444
- 全文大小：166K
- 年卷期：v.46,no.28(July 17, 2007)
- ISSN：1520-4995
文摘
Glucosyltransferases (GTFs) secreted by mutans streptococci and some other lactic acid bacteriacatalyze glucan synthesis from sucrose, and possess a C-terminal glucan-binding domain (GBD) containinghomologous, directly repeating units. We prepared a series of C-terminal truncated forms of the GBD ofStreptococcus sobrinus GTF-I and studied their binding to dextran by isothermal titration calorimetry.The binding of all truncates was strongly exothermic. Their titration curves were analyzed assuming thatthe GBD recognizes and binds to a stretch of dextran chain, not to a whole dextran molecule. Both thenumber of glucose units constituting the dextran stretch (n) and the accompanying enthalpy change (
H
)are proportional to the molecular mass of the GBD truncate, with which the Gibbs energy change calculatedby the relation G = -RT ln K (R, the gas constant; T, the absolute temperature; K, the binding constantof a truncate for a dextran stretch of n glucose units) also increases linearly. For the full-length GBD (508amino acid residues), n = 33.9, K = 4.88 × 107 M-1, and H = -289 kJ mol-1 at 25 C. These resultssuggest that identical, independent glucose-binding subsites, each comprising 14 amino acid residues onaverage, are arranged consecutively from the GBD N-terminus. Thus, the GBD binds tightly to a stretchof dextran chain through the adding up of individually weak subsite/glucose interactions. Furthermore,the entropy change accompanying the GBD/dextran interaction as given by the relation S = (G -H)/T has a very large negative value, probably because of a loss of the conformational freedom ofdextran and GBD after binding.
H)are proportional to the molecular mass of the GBD truncate, with which the Gibbs energy change calculatedby the relation G = -RT ln K (R, the gas constant; T, the absolute temperature; K, the binding constantof a truncate for a dextran stretch of n glucose units) also increases linearly. For the full-length GBD (508amino acid residues), n = 33.9, K = 4.88 × 107 M-1, and H = -289 kJ mol-1 at 25 C. These resultssuggest that identical, independent glucose-binding subsites, each comprising 14 amino acid residues onaverage, are arranged consecutively from the GBD N-terminus. Thus, the GBD binds tightly to a stretchof dextran chain through the adding up of individually weak subsite/glucose interactions. Furthermore,the entropy change accompanying the GBD/dextran interaction as given by the relation S = (G -H)/T has a very large negative value, probably because of a loss of the conformational freedom ofdextran and GBD after binding.