Role of the N-Terminal Hydrophilic Domain of Acyl-Coenzyme A:Cholesterol Acyltransferase 1 on the Enzyme's Quaternary Structure and Catalytic Efficiency
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- 作者：Chunjiang Yu ; Yi Zhang ; Xiaohui Lu ; Jun Chen ; Catherine C. Y. Chang ; and Ta-Yuan Chang
- 刊名：Biochemistry
- 出版年：2002
- 出版时间：March 19, 2002
- 年：2002
- 卷：41
- 期：11
- 页码：3762 - 3769
- 全文大小：162K
- 年卷期：v.41,no.11(March 19, 2002)
- ISSN：1520-4995
文摘
Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an enzyme involved in cellularcholesterol homeostasis and atherosclerosis. ACAT1 is an allosteric enzyme responding to its substratecholesterol in a sigmoidal manner. It is a homotetrameric protein that spans the membrane multiple times,with its N-terminal 131 hydrophilic amino acids residing at the cytoplasmic side of the endoplasmicreticulum. This region contains two closely linked putative 
-helices. Our current studies show that thisregion contains a dimer-forming motif. Adding this motif to the bacterial glutathione S-transferase (GST)converted the homodimeric GST to a tetrameric fusion protein. Conversely, deleting this motif from thefull-length ACAT1 converted the enzyme from a homotetramer to a homodimer. The dimeric ACAT1remains enzymatically active. Its biochemical characteristics, including the sigmoidal response to cholesterol,the IC50 value toward a specific ACAT inhibitor, and sensitivity toward heat inactivation, are essentiallyunaltered. On the other hand, the dimeric ACAT1 exhibits a 5-10-fold increase in the Vmax of the overallreaction and a 2.2-fold increase in the Km for oleoyl-coenzyme. Thus, deleting the dimer-forming motifnear the N-terminus changes ACAT1 from its tetrameric form to a dimeric form and increases its catalyticefficiency.
-helices. Our current studies show that thisregion contains a dimer-forming motif. Adding this motif to the bacterial glutathione S-transferase (GST)converted the homodimeric GST to a tetrameric fusion protein. Conversely, deleting this motif from thefull-length ACAT1 converted the enzyme from a homotetramer to a homodimer. The dimeric ACAT1remains enzymatically active. Its biochemical characteristics, including the sigmoidal response to cholesterol,the IC50 value toward a specific ACAT inhibitor, and sensitivity toward heat inactivation, are essentiallyunaltered. On the other hand, the dimeric ACAT1 exhibits a 5-10-fold increase in the Vmax of the overallreaction and a 2.2-fold increase in the Km for oleoyl-coenzyme. Thus, deleting the dimer-forming motifnear the N-terminus changes ACAT1 from its tetrameric form to a dimeric form and increases its catalyticefficiency.