engleski

pH-induced changes of Apob-100 studied by ir spectroscopy

Lipoproteins consists of a nonpolar core where triacylglycerols and cholesteryl esters are hidden surrounded by a monolayer facing the water composed of phospholipid, cholesterol and proteins, giving these lipid-rich structures water solubility. Blood plasma lipoproteins are classified on the basis of their density, which in turn is a reflection of their lipid content. The greater their lipid contents the lower their density. Three major lipoprotein classes are found in fasting human blood plasma, VLDL (very low density lipoprotein), LDL (low density lipoprotein) and HDL (high density lipoprotein). Low density lipoprotein is the principal carrier of cholesterol and play an important role in the development of atherosclerotic lesions. It is described as a spherical particle containing a hydrophobic core of cholesteryl esters and triglycerides surrounded by an amphipatic monolayer of phospholipid and cholesterol in which a single molecule of protein (apoB) is located. ApoB is one of the largest proteins known and is extremely insoluble in aqueous media. This has hindered the understanding of the structural basis of apoB function. Infrared spectroscopy has been applied to characterize the secondary structure of apoB and its topology in the LDL particle. Lipoproteins, the major carriers of lipids in human plasma, are a good topic to study by infrared spectroscopy. The components of the particles, lipids and proteins, give rise to independent bands in the infrared spectrum that allow the study of both moieties in the same sample. Structural analysis usually implies a mathematical approach in order to extract the information contained in the composite bands, designated in IR spectroscopy as amide bands, obtained from proteins. Commonly used methods of analysis imply narrowing the intrinsic bandwidths to visualize the overlapping band components and then decomposing the original band contour into these components by means of an iterative process. The various components are finally assigned to protein or subunit structural features. Changes in the protein components of LDL play a substantial role in the initiation and progression of atherosclerosis. In the atheroma, pH is low, far from the physiological values, being a factor that can induce conformational changes in apoB-100 related with the atherogenic properties of the particle. We have measured the amide I band at pH 4.5 proving that a change characterized by a stronger intensity at 1650 cm-1 is produced by acidification. Restoration of pH at 7.4 recovers the original shape. The changes and its reversibility have been corroborated by thermal profiles and 2D-IR spectroscopy.

LDL; pH; apoB

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