Structure of aldose reductase from Giardia lamblia

Abstract

Giardia lamblia is an anaerobic aerotolerant eukaryotic parasite of the intestines. It is believed to have diverged early from eukarya during evolution and is thus lacking in many of the typical eukaryotic organelles and biochemical pathways. Most conspicuously, mitochondria and the associated machinery of oxidative phosphorylation are absent; instead, energy is derived from substrate-level phosphorylation. Here, the 1.75 Å resolution crystal structure of G. lamblia aldose reductase heterologously expressed in Escherichia coli is reported. As in other oxidoreductases, G. lamblia aldose reductase adopts a TIM-barrel conformation with the NADP(+)-binding site located within the eight β-strands of the interior.

Figure 1

Dimer of aldose reductase from G. lamblia (GilaA.01452.aA1). Aldose reductase from G. lamblia forms a dimer of two TIM barrels. In this figure the twofold axis runs approximately vertically. The helices of the two monomers are shown as blue and purple ribbons. In each momomer, an NADP molecule is visible at the C-­terminal end of the strands of the β-barrel. This figure was prepared with CCP4mg (McNicholas et al., 2011 ▶).

Figure 2

(a) NADP is well defined in the structure of G. lamblia aldose reductase. (b) The adenine ring of NADP stacks hydrophobically with the side chains of His242 and Arg269. While Arg269 is conserved between human and Giardia aldose reductase, His242 of Giardia aldose reductase is an alanine in the human enzyme. This figure was prepared with CCP4mg (McNicholas et al., 2011 ▶). In both cases σ_A-weighted 2|F _o| − |F _c| electron density is shown in blue at 1σ and σ_A-weighted |F _o| − |F _c| electron density is shown in green and red at ±3σ.

Figure 3

ClustalW (Larkin et al., 2007 ▶) comparison of the amino-acid sequences of H. sapiens and G. lamblia aldose reductase.

Figure 4

Inhibitor-binding pocket of G. lamblia aldose reductase in comparison with ligand-bound human and porcine aldose reductases. G. lamblia aldose reductase (red model) superimposes with small r.m.s.d.s with human aldose reductase in complex with fidarestat (PDB entry 1ef3, dark blue) and porcine aldose reductase in complex with tolrestat (PDB entry 1ah3, green) and sorbinil (PDB entry 1ah0, cyan). Despite the overall structural homology, the C-terminal loop, which interacts with the ligand, shows a distinctly different trace. For clarity, the C atoms of the inhibitors follow the same color scheme as the corresponding ribbon. The NADP molecule is shown only for G. lamblia, with light green C atoms. This figure was prepared with CCP4mg (McNicholas et al., 2011 ▶).