MCAT Biochemistry Review

Chapter 11: Lipid and Amino Acid Metabolism

11.3 Lipid Transport

Figure 11.3. Lipoprotein Structure

While free fatty acids are transported through the blood in association with albumin, a carrier protein, triacylglycerol and cholesterol are transported in the blood as lipoproteins: aggregates of apolipoproteins and lipids, as shown in Figure 11.3. Lipoproteins are named according to their density, which increases in direct proportion to the percentage of protein in the particle. Chylomicrons are the least dense, with the highest fat-to-protein ratio. VLDL (very-low-density lipoprotein) is slightly more dense, followed by IDL (intermediate-density), LDL (low-density), and HDL (high-density). The main functions of each lipoprotein are shown in Table 11.1. Note that chylomicrons and VLDL primarily carry triacylglycerols, but also contain small quantities of cholesteryl esters. LDL and HDL are primarily cholesterol transport molecules.

Lipoprotein

Functions

Chylomicrons

Transport dietary triacylglycerols and cholesterol from intestine to tissues

VLDL

Transports triacylglycerols from liver to tissues

IDL (VLDL remnants)

Picks up cholesterol from HDL to become LDL Picked up by the liver

LDL

Delivers cholesterol into cells

HDL

Picks up cholesterol accumulating in blood vessels Delivers cholesterol to liver and steroidogenic tissues Transfers apolipoproteins to other lipoproteins

Table 11.1. Classes of Lipoproteins

CHYLOMICRONS

Chylomicrons are highly soluble in both lymphatic fluid and blood and function in the transport of dietary triacylglycerol, cholesterol, and cholesteryl esters to other tissues. Assembly of chylomicrons occurs in the intestinal lining and results in a nascent chylomicron that contains lipids and apolipoproteins.

VLDL (VERY-LOW-DENSITY LIPOPROTEIN)

VLDL metabolism is similar to that of chylomicrons; however, VLDL is produced and assembled in liver cells. Like chylomicrons, the main function of VLDL is the transport of triacylglycerol to other tissues. VLDLs also contain fatty acids that are synthesized from excess glucose or retrieved from chylomicron remnants.

IDL (INTERMEDIATE-DENSITY LIPOPROTEIN)

Once triacylglycerol is removed from VLDL, the resulting particle is referred to as either a VLDL remnant or IDL. Some IDL is reabsorbed by the liver by apolipoproteins on its exterior, and some is further processed in the bloodstream. For example, some IDL picks up cholesteryl esters from HDL to become LDL. IDL thus exists as a transition particle between triacylglycerol transport (associated with chylomicrons and VLDL) and cholesterol transport (associated with LDL and HDL). This process is shown in Figure 11.4.

Figure 11.4. Lipid Transport in Lipoproteins TGL = triacylglycerol; CE = cholesteryl esters; chol = cholesterol.

KEY CONCEPT

Chylomicrons and VLDL primarily carry triacylglycerols. LDL and HDL primarily carry cholesterol. IDL is intermediate; it is a transition state between VLDL and LDL, occurring as the primary lipid within the lipoprotein changes from triacylglycerol to cholesterol.

LDL (LOW-DENSITY LIPOPROTEIN)

Although both LDL and HDL are primarily cholesterol particles, the majority of the cholesterol measured in blood is associated with LDL. The normal role of LDL is to deliver cholesterol to tissues for biosynthesis. However, cholesterol also plays an important role in cell membranes. In addition, bile acids and salts are made from cholesterol in the liver, and many other tissues require cholesterol for steroid hormone synthesis (steroidogenesis).

HDL (HIGH-DENSITY LIPOPROTEIN)

HDL is synthesized in the liver and intestines and released as dense, protein-rich particles into the blood. HDL contains apolipoproteins used for cholesterol recovery—that is, the cleaning up of excess cholesterol from blood vessels for excretion. HDL also delivers some cholesterol to steroidogenic tissues and transfers necessary apolipoproteins to some of the other lipoproteins.

REAL WORLD

When a physician orders a blood test for cholesterol, they are actually measuring levels of LDL and HDL in the blood. HDL is often considered “good” cholesterol because it picks up excess cholesterol from blood vessels for excretion.

APOLIPOPROTEINS

Apolipoproteins, also referred to as apoproteins, form the protein component of the lipoproteins described above. Apolipoproteins are receptor molecules and are involved in signaling. While it is highly unlikely that specific functions of each apolipoprotein will be tested on the MCAT, they are briefly summarized below to illustrate their diverse purposes:

·        apoA-I: activates LCAT, an enzyme that catalyzes cholesterol esterification

·        apoB-48: mediates chylomicron secretion

·        apoB-100: permits uptake of LDL by the liver

·        apoC-II: activates lipoprotein lipase

·        apoE: permits uptake of chylomicron remnants and VLDL by the liver

MCAT Concept Check 11.3:

Before you move on, assess your understanding of the material with these questions.

1.    What is the primary method of transporting free fatty acids in the blood?

2.    Order the lipoproteins from greatest percentage of protein to least percentage of protein. Circle the molecules that are primarily involved in triacylglycerol transport.

3.    Lipoproteins are synthesized primarily by which two organs?