Crystallographic refinement of human serum retinol binding protein at 2Å resolution

Abstract

Human serum retinol binding protein (RBP) in complex with retinol has been crystallographically refined to an R-factor of 18.1% with 2A resolution data. The protein topology results in an anti-parallel beta-barrel that encapsulates the retinol ligand. A detailed description of the protein and the binding site is provided. Our structural work has helped to define a family of proteins, many of which are carrier proteins for smaller ligand molecules. We describe the structural basis for the conservation of sequence within the family.

Mechanism of pH‐induced release of retinol from retinol‐binding protein

“…Patterns of charged groups have been studied using X-ray atomic coordinates of human hoIoRBP in the orthorhombic crystal form (resolution 1.9 A) [2], human holo and apoRBP* in the trigonal form (resolution 2.5 A) [3] and bovine holo and apoRBP (resolution 1.9 and 1.7 A, respectively) [4] residues. These four salt bridges, listed in Table I, are established from X-ray coordinates of human holo and upoRBP [2,3] and bovine holo and apoRBP [4].…”

“…These four salt bridges, listed in Table I, are established from X-ray coordinates of human holo and upoRBP [2,3] and bovine holo and apoRBP [4]. In addi- [2].…”

“…These four salt bridges, listed in Table I, are established from X-ray coordinates of human holo and upoRBP [2,3] and bovine holo and apoRBP [4]. In addi- [2]. This picture is not changed by the pattern of nonconservative charged residues: in mammalian RBPs and in X. Levis there are two other positive charges (Arg-62' and Lys/Arg-99') and one negative charge around the hole.…”

The lipocalin protein family: structure and function

“…In contrast with their low conservation at the sequence level, analysis of available lipocalin crystal structures, which include plasma retinol-binding protein (RBP) [11], β-lactoglobulin (Blg) [12], insecticyanin [13], bilin-binding protein (BBP) [14,15], major urinary protein (MUP) and α #u -globulin [16], odorant-binding protein (OBP) [17] and epididymal retinoic acid-binding protein [18], shows that the overall folding pattern common to the lipocalins is highly conserved. The nature of this common structure is now well described (see Figures 1 and 2) [2,11]. The lipocalin fold is a highly symmetrical all-β protein dominated by a single eight-stranded antiparallel β-sheet closed back on itself to form a continuously hydrogen-bonded β-barrel, which, in cross-section, has a flattened or elliptical shape and encloses an internal ligand-binding site.…”

“…In this regard they are typified by RBP [11,35], virtually the sole retinol transporter in plasma, which binds a single all-trans-retinol molecule as its physiological ligand. RBP is synthesized in hepatic parenchymal cells, where the apoprotein is saturated with retinol, triggering its secretion into general circulation.…”