Chapter Introduction
The Biosynthesis of Membrane Lipids and Steroids
Fats such as the triacylglycerol molecule (above right) are widely used to store excess energy for later use and to fulfill other purposes, illustrated by the insulating blubber of whales. The natural tendency of fats to exist in nearly water free forms makes these molecules well suited to these roles.
OUTLINE
Phosphatidate Is a Common Intermediate in the Synthesis of Phospholipids and Triacylglycerols
Cholesterol Is Synthesized from Acetyl Coenzyme A in Three Stages
The Complex Regulation of Cholesterol Biosynthesis Takes Place at Several Levels
Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
This chapter examines the biosynthesis of three important components of biological membranes—phospholipids, sphingolipids, and cholesterol—which were introduced in Chapter 12. Triacylglycerols also are considered here because the pathway for their synthesis overlaps that of phospholipids. Cholesterol is of interest both as a membrane component and as a precursor of potent signal molecules, such as the steroid hormones progesterone, testosterone, estrogen, and cortisol.
The transport and uptake of cholesterol vividly illustrate a recurring mechanism for the entry of metabolites and signal molecules into cells. Cholesterol is transported in blood by the low-density lipoprotein (LDL) and taken up into cells by the LDL receptor on the cell surface. The LDL receptor is absent in people with familial hypercholesterolemia, a genetic disease. People lacking the receptor have markedly elevated cholesterol levels in the blood and cholesterol deposits on blood vessels, and they are prone to childhood heart attacks. Indeed, cholesterol is implicated in the development of atherosclerosis in people who do not have genetic defects. Thus, the regulation of cholesterol synthesis and transport can be a source of especially clear insight into the role that our understanding of biochemistry plays in medicine.