Summary: | 碩士 === 國立成功大學 === 生物化學研究所 === 84 === Aeromonas hydrophila分泌一個由865個胺基酸組成,分子量約94
kDa的幾丁分解酵素,此酵素之蛋白質結構主要是由N端訊息序列,催化區
域以及含有兩個重覆序列區域的C端 (93個胺基酸) 所組成;而位於C端的
二個重覆序列 (CICII) 是具有51%相同性的35個胺基酸所構成。藉由蛋白
質序列相似性比對分析顯示,幾丁質分解酵素的兩重覆序列區域和
Bacillus sp. strain N-4的纖維水解酵素的C端區域有很高的相似性。
在結合作用的研究發現,C端截短的幾丁分解酵素及C端 (CICII) 融合
到glutathione S-transferase (GST) 的融合蛋白均顯示C端重覆序列能
調控幾丁分解酵素對膠狀幾丁質及纖維素的結合能力,且GST-CI (或GST-
CII) 的融合蛋白本身即可和膠狀幾丁質進行結合作用。在上述的蛋白質
序列比對分析,發現有八個Trp是最具保存性的胺基酸,分別是Trp-773、
Trp-792、Trp-897、Trp-810、Trp-820、Trp-839、Trp-844以及Trp-856
;將Trp-792以定點突變法將Trp (W)突變為Gly (G),和野生株做幾丁質
結合能力的比較,發現不管是兩重覆序列或單一重覆序列結合區域的突變
株 (GST-CICIIW792G或GST-CIW792G),其幾丁質結合能力分別比野生株
(GST-CICII或GST-CI) 約下降20-30%。 在幾丁分解酵素活性輔助因子
的研究中,發現在幾丁分解酵素活性突變株中,幾丁分解酵素結構基因上
游500 bp以及下游3 kb是影響酵素是否具有活性的重要因素。
Chitinase (ChiA) secreted by Aeromonas hydrophila is a
865-amino-acid (aa) polypeptide composed of an N-terminal signal
peptide, a catalytic domain and a C-terminal chitin-binding
domain containing two subdomains (CI and CII) which showed
strong similarity (51% identity). By comparison with other
glycosyl hydrolases, the C-terminal subdomains (CI or CII)
showed significant similarity to C-terminal region of cellulase
from Bacillus sp. strain N-4. In binding studies, intact ChiA
binded strongly not only to colloidal chitin but also to
cellulose, while the truncated ChiA derivatives lacking the C-
terminal domain absorbed weakly to the colloidal chitin and
cellulose. When CI-CII was fused to glutathione S-transferase
(GST), it exhibited the binding property to colloidal chitin and
cellulose, and furthermore, GST-CI (or GST-CII) also showed the
signficant chitin-binding property. These observations indicated
that C-terminal subdomains modulate the binding character of
ChiA to colloidal chitin and cellulose. There are eight Trp
residues (Trp-773, Trp-792, Trp-797, Trp-810, Trp-820, Trp-839,
Trp-844 and Trp-856 in ChiA) conserved between the two C-termini
of the A .hydrophila chitinase subdomains and the Bacillus
cellulase. Among them, Trp-792 was chosen as the target for the
amino acid substitution (Trp ?Gly) by site-directed
mutagenesis. The results of binding experiments showed that the
binding activites of the two mutants, GST-CICIIW792G and GST-
CIW792G, were reduced to 70-80% when compared to those of GST-
CICII and GST-CI, respectively. In the study of enzyme
activity assistant factors for chitinase by using E. coli
mutant, it was found that both the 0.5 kb upstream sequence and
3 kb downstream sequence may play an important role in
determining the formation of the active form of chitinase.
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