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COVER: Comparison of the side chain organization of selected amino acid residues in the LDL receptor-binding domain of the three common isoforms of human apoE. The -helix backbones are shown as strands and the amino acid side chains are drawn as balls and sticks. Lipoprotein particles containing apoE2 bind much more weakly to the LDL receptor than particles containing apoE3 or apoE4. In the latter isoforms, the negative charge on aspartic acid 154 is neutralized by arginine 158, allowing high-affinity binding to the LDL receptor via basic residues located between positions 140 and 150 on the polar face of helix 4. The decreased binding of apoE2 is due to a reduced positive electrostatic potential between residues 140 and 150; this occurs because, with cysteine at position 158, aspartic acid 154 neutralizes arginine 150. This causes the reduction in positive electrostatic potential, which is reflected in an increased pKa of lysine 146, as measured by NMR. (See Lund-Katz et al., p. 894)
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