Four Major Lipid Classes Are Present in Lipoproteins

Plasma lipids consist of triacylglycerols (16%), phospholipids (30%), cholesterol (14%), and cholesteryl esters (36%) and a much smaller fraction of unesterified long-chain fatty acids (or FFAs) (4%). This latter fraction, the FFA, is metabolically the most active of the plasma lipids.

Four Major Groups of Plasma Lipoproteins

Have Been Identified Since fat is less dense than water, the density of a lipoprotein decreases as the proportion of lipid to protein increases (Table 1). Four major groups of lipoproteins have been identified that are important physiologically and in clinical diagnosis.

Table1. Composition of the Lipoproteins in Plasma of Humans

These are (1) chylomicrons, derived from intestinal absorption of triacylglycerol and other lipids; (2) VLDL, derived from the liver for the export of triacylglycerol; (3) low-density lipoproteins (LDL), responsible for the transport of cholesterol in humans and representing a final stage in the catabolism of VLDL; and (4) high-density lipoproteins, (HDL), involved in reverse cholesterol transport and also in VLDL and chylomicron metabolism. Triacylglycerol is the predominant lipid in chylomicrons and VLDL, whereas cholesterol and phospholipid are the predominant lipids in LDL and HDL, respectively (see Table 1). Lipoproteins may also be classified according to their electrophoretic properties into α- (HDL), β- (LDL), and pre-β (VLDL)-lipoproteins.

Lipoproteins Consist of a Nonpolar Core & a Single Surface Layer of Amphipathic Lipids

The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phosphor lipid and cholesterol molecules (Figure 1). These are oriented so that their polar groups face outward to the aqueous medium, as in the cell membrane. The protein moiety of a lipoprotein is known as an apolipoprotein or apoprotein, constituting nearly 70% of some HDL and as little as 1% of chylomicrons.

Fig1. Generalized structure of a plasma lipoprotein. Small amounts of cholesteryl ester and triacylglycerol are found in the surface layer and a little free cholesterol in the core.

The Distribution of Apolipoproteins Characterizes the Lipoprotein One or more apolipoproteins are present in each lipoprotein. They are usually abbreviated as apo followed by the letters A, B, C, etc. (see Table1). Some apolipoproteins are integral and cannot be removed (eg, apo B), whereas others are bound to the surface and are free to transfer to other lipoproteins (eg, apos C and E). The major apolipoproteins of HDL (α-lipoprotein) are apo As (see Table 1). The main apolipoprotein of LDL (β-lipoprotein) is apo B (B-100), which is found also in VLDL. Chylomicrons contain a truncated form of apo B-100, termed apoB-48 as it constitutes 48% of the full-sized molecule. Apo B-48 is synthesized in the intestine, while B-100 is synthesized in the liver. Apo B-100 is one of the longest single polypeptide chains known, having more than 4500 amino acids and a molecular mass of 540,000 Da. To produce Apo B-48, a stop signal is introduced into the mRNA transcript for apo B-100 by an RNA editing enzyme. Apos C-I, C-II, and C-III are smaller polypeptides (molecular mass 7000-9000 Da) freely transferable between several different lipoproteins. Apo E, found in VLDL, HDL, chylomicrons, and chylomicron remnants, is also freely transferable; it accounts for 5 to 10% of total VLDL apolipoproteins in normal subjects.

Apolipoproteins carry out several roles: (1) they can form part of the structure of the lipoprotein, for example, apo B; (2) they are enzyme cofactors, for example, C-II for lipoprotein lipase, A-I for lecithin: cholesterol acyltransferase (LCAT), or enzyme inhibitors, for example, apo A-II and apo C-III for lipoprotein lipase, apo C-I for cholesteryl ester transfer protein; and (3) they act as ligands for interaction with lipoprotein receptors in tissues, for example, apo B-100 and apo E for the LDL receptor, apo E for the LDL-receptor related protein-1 (LRP-1), which recognizes remnant lipoproteins , and apo A-I for the HDL receptor. Apo A-IV is thought to have a role in chylomicron metabolism and may also act as a regulator of satiety and glucose homeostasis, making it a potential therapeutic target for the treatment of diabetes and obesity, while apo D is believed to be an important factor in human neurodegenerative disorders.

اخر الاخبار

اخبار العتبة العباسية المقدسة

استعدادًا لزيارة الأربعين العتبة العباسية المقدسة تعقد اجتماعًا لتنظيم عمل الهيئات التطوّعية في الصحن الشريف

قسم الشؤون الخدمية يواصل تنفيذ أعمال الوقاية ومكافحة الحشرات في مواقع العتبة العباسية المقدسة

المجمَع العلمي للقرآن الكريم يواصل تنفيذ مشروع بيوت النور القرآني

قسم الشؤون الفكرية يبحث التعاون في مجال البحث مع كلية التقنيات المتعددة في نيجيريا

المزيد

الآخبار الصحية

ماذا يحدث لضغط الدم مع تناول بذور الشيا؟

الأولى من نوعها.. حقنة ثورية تنهي معاناة فقدان السمع نهائيا !

7 علامات تدل على نقص معدن مهم في الجسم.. تعرف عليها

الجسم يستعيد الوزن الذي فقده بعد أسابيع من وقف أدوية التنحيف

المزيد

الآخبار التكنلوجية

أمواج تسونامي تصل إلى هاواي بعد زلزال كامتشاتكا !

تشات جي بي تي يتجاوز اختبار "أنا لست روبوتا" المرعب ؟!

أستراليا تعتزم حظر استخدام "يوتيوب" لهذه الفئة العمرية

دراسة تكشف تأثير استخدام الهواتف بسن مبكرة على الصحة النفسية

المزيد

مواضيع ذات صلة

Brown Adipose Tissue Promotes Thermogenesis

Hormones Regulate Fat Mobilization

Adipose Tissue is The Main Store of Triacylglycerol in The Body

Classification of Proteins

Precipitation Reactions of Proteins

Study of Protein Structure

Imbalance in the Rate of Triacylglycerol Formation & Export Causes Fatty Liver

The Liver Plays A Central Role in Lipid Transport & Metabolism

HDL Takes Part in Both Lipoprotein Triacylglycerol & Cholesterol Metabolism

Tertiary Structure of Proteins

Secondary Structure of Proteins

Primary Structure of Proteins