Vertebrates obtain vitamin A from the diet either as carotenoids or as long chain fatty acid esters of retinol which, prior to absorption, are in both cases converted to free retinol, the form of the vitamin taken up by the intestinal mucosa.1 After cellular uptake, the free retinol is bound by cellular retinol-binding proteins (CRBPs), a protein family that in mammals includes four characterized members with significant sequence similarity but a very different tissue distribution.2 In particular, the form that is most abundant in the absorbing mucosa of the small intestine is cellular retinol binding protein II (CRBP II), which accounts for ∼1% of the total soluble protein present in this tissue. The 133 amino acid CRBP II is believed to play several very important roles in the metabolism of the vitamin3–5: it can bind to the membrane transporters to facilitate retinol entrance into the cell, it protects the cell from the detergent effects of free retinol and probably interacts with the enzymes that re-esterify retinol prior to its release into the chylomicrons and thus modulates retinol metabolism.6 The protein was first purified from rat tissue,7 the species in which this molecule has been most extensively studied. The X-ray structure of both the apo and holo forms of rat CRBP II to 2 Å resolution revealed that there were essentially no conformational changes between the apo and holo forms.8 Subsequent NMR experiments have addressed this issue in very great detail.9–11 More recently, a CRBP highly homologous to rat CRBP II was identified and structurally characterized in the zebrafish. In this case, the resolution of the crystal structures was higher, 1.4 Å for the holo and 1.7 Å for the apo form, and more significant variations were observed between the two forms.12 Human CRBP II was first isolated and partially characterized many years after the discovery of rat CRBP II13 and it was later shown that retinol uptake and metabolism are correlated to its level in the human intestinal cells.14 We report here the X-ray structures of human apo and holo CRBP II solved at 1.2 Å resolution and compare the two structures between them and with the structures of zebrafish and rat CRBP II.
Crystal structure of human cellular retinol-binding protein II to 1.2 Å resolution
CAPALDI, Stefano;AMBROSI, Emanuele;PERDUCA, Massimiliano;MONACO, Ugo Luigi
2008-01-01
Abstract
Vertebrates obtain vitamin A from the diet either as carotenoids or as long chain fatty acid esters of retinol which, prior to absorption, are in both cases converted to free retinol, the form of the vitamin taken up by the intestinal mucosa.1 After cellular uptake, the free retinol is bound by cellular retinol-binding proteins (CRBPs), a protein family that in mammals includes four characterized members with significant sequence similarity but a very different tissue distribution.2 In particular, the form that is most abundant in the absorbing mucosa of the small intestine is cellular retinol binding protein II (CRBP II), which accounts for ∼1% of the total soluble protein present in this tissue. The 133 amino acid CRBP II is believed to play several very important roles in the metabolism of the vitamin3–5: it can bind to the membrane transporters to facilitate retinol entrance into the cell, it protects the cell from the detergent effects of free retinol and probably interacts with the enzymes that re-esterify retinol prior to its release into the chylomicrons and thus modulates retinol metabolism.6 The protein was first purified from rat tissue,7 the species in which this molecule has been most extensively studied. The X-ray structure of both the apo and holo forms of rat CRBP II to 2 Å resolution revealed that there were essentially no conformational changes between the apo and holo forms.8 Subsequent NMR experiments have addressed this issue in very great detail.9–11 More recently, a CRBP highly homologous to rat CRBP II was identified and structurally characterized in the zebrafish. In this case, the resolution of the crystal structures was higher, 1.4 Å for the holo and 1.7 Å for the apo form, and more significant variations were observed between the two forms.12 Human CRBP II was first isolated and partially characterized many years after the discovery of rat CRBP II13 and it was later shown that retinol uptake and metabolism are correlated to its level in the human intestinal cells.14 We report here the X-ray structures of human apo and holo CRBP II solved at 1.2 Å resolution and compare the two structures between them and with the structures of zebrafish and rat CRBP II.
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