Cholesterol plays essential roles in your body, as a structural component of cell membranes, as well as a precursor of steroid hormones, vitamin D and bile acids. Nearly all tissues in humans can produce cholesterol and, according to the Merck Manual, your body has the ability to make all the cholesterol it needs. Whether it originates from your diet or your own body's production, cholesterol essentially maintains the same composition.
Chemical Structure
Scientists Pamela Champe and Richard Harvey describe cholesterol as the major steroid alcohol, or sterol, in animal tissues. Its characteristic feature is a steroid nucleus consisting of four fused carbon rings: three hexagonal rings named A, B and C; and a pentagonal ring D. Attached to the D ring is an eight-carbon side chain. Champe and Harvey also note that ring A contains an alcohol, or hydroxyl, group, while ring B has a double carbon bond. The above-named features of cholesterol's structure make it very hydrophobic, or insoluble in liquids..
Common Forms
The majority of the cholesterol in your blood exists as cholesteryl ester, which is even more hydrophobic than free cholesterol. According to Champe and Harvey, cholesteryl esters are not present in your cell membranes, and only exist in low levels in most cells. Cholesteryl esters only differ from free cholesterol in that they are bound to fatty acids. Because of its water-hating character, cholesterol and its esters cannot freely circulate in the blood. Rather, they must either be packaged into lipoproteins, or be solubilized in the bile. Bile is the single most significant thing your body uses to get rid of excess cholesterol.
Transport
You may have heard of HDL and LDL being referred to as "good" and "bad" cholesterol, respectively. This can, however, be misleading, because LDL and HDL are not cholesterol types. Rather, they belong to a family of molecules that carry cholesterol and other lipids, to enable their circulation in the blood. LDL stands for "low-density lipoprotein," while HDL stands for "high-density lipoprotein." LDL cholesterol earned its reputation of "bad cholesterol" from the fact that the LDL molecules carries cholesterol from the liver to your arteries and organs, thereby increasing the chances of cholesterol accumulating there. HDL, on the other hand, removes cholesterol from arteries and returns it to your liver.
Lipoprotein Structure
Lipoproteins are spherical complexes consisting of a lipid core surrounded by a shell of proteins, phospholipids and non-esterified cholesterol. The lipid core contains cholesteryl esters or other fats. To make the complex soluble in blood, the surrounding shell is arranged so that the water-loving portions of its phospholipids and proteins are exposed on the surface of the lipoprotein. Pharmacologists Anthony Trevor and Bertram Katzung describe cholesterol and saturated fats as the main dietary factors that increase blood levels of lipoproteins. However, the cholesterol your lipoproteins carry can also come from your body's production. This is because as your body detects a higher cholesterol content, it ups its production of lipoproteins to facilitate transport.
Synthesis
Cholesterol synthesis begins with a two-carbon molecule called acetate. Acetate is a salt or an ester of acetic acid, a compound you may recognize as the acid that gives vinegar its sour taste. Through a series of reactions involving many other reagents, your body converts acetate into HMG-CoA, and then into mevalonate. The conversion of HMG-CoA into mevalonate is a key step, because it determines the overall rate at which your body produces cholesterol. As a result, this is the step many cholesterol-lowering drugs are designed to block, according to Trevor and Katzung. Subsequent reactions lead to ianosterol--a parent compound of cholesterol--and finally to cholesterol.
Considerations
Nearly all tissues in your body produce cholesterol, although the bulk of this production occurs in the liver and reproductive tissues. Champe and Harvey indicate that the human body works hard to maintain a balance by producing only the amount of cholesterol it needs, and excreting any excess. However, a cholesterol-rich, or high-fat diet, can overwhelm your body. As cholesterol accumulates in your bloodstream, the risk of plaque formation in your arteries increases, according to the Merck Manual. This in turn may contribute to heart disease. Other reasons for imbalances in blood cholesterol include aging, various hereditary conditions, certain medications, a lack of physical exercise or being overweight.
References
- "Lippincott's Illustrated Reviews: Biochemistry"; Richard Harvey, Ph.D., and Pamela Champe, Ph.D.; 2005
- The Merck Manual: Cholesterol Disorders
- "Katzung & Trevor's Pharmacology Examination & Board Review"; Anthony Trevor, Ph.D., Bertram Katzung, Ph.D.; 2008
