Sterol O-acyltransferase

Sterol O-acyltransferase (also called Acyl-CoA cholesterol acyltransferase, Acyl-CoA cholesterin acyltransferase or simply ACAT) is an intracellular protein located in the endoplasmic reticulum that forms cholesteryl esters from cholesterol.

Sterol O-acyltransferase catalyzes the chemical reaction:

acyl-CoA + cholesterol

⇌

{\displaystyle \rightleftharpoons }

CoA + cholesterol ester

Thus, the two substrates of this enzyme are acyl-CoA and cholesterol, whereas its two products are CoA and cholesteryl ester.

This enzyme belongs to the family of transferases, specifically those acyltransferases transferring groups other than aminoacyl groups, the membrane-bound O-acyltransferases. This enzyme participates in bile acid biosynthesis.

Class and structure

Acyl-CoA cholesterol acyl transferase EC 2.3.1.26, more simply referred to as ACAT, also known as sterol O-acyltransferase (SOAT), belongs to the class of enzymes known as acyltransferases. The role of this enzyme is to transfer fatty acyl groups from one molecule to another. ACAT is an important enzyme in bile acid biosynthesis.

In nearly all mammalian cells, ACAT catalyzes the intracellular esterification of cholesterol and formation of cholesteryl esters. The esterification of cholesterol mediated by ACAT is functionally significant for several reasons. ACAT-mediated esterification of cholesterol limits its solubility in the cell membrane lipids and thus promotes accumulation of cholesterol ester in the fat droplets within cytoplasm; this process is important because the toxic accumulation of free cholesterol in various cell membrane fractions is prevented.