Preliminary studies on the primary structure of rat liver dihydropteridine reductase

Crystal structure of the ternary complex of mouse lung carbonyl reductase at 1.8 A resolution: the structural origin of coenzyme specificity in the short-chain dehydrogenase/reductase family

BACKGROUND

Mouse lung carbonyl reductase (MLCR) is a member of the short-chain dehydrogenase/reductase (SDR) family. Although it uses both NADPH and NADH as coenzymes, the structural basis of its strong preference for NADPH is unknown.

RESULTS

The crystal structure of the ternary complex of MLCR (with NADPH and 2-propanol) has been determined at 1.8 A resolution. This is the first three-dimensional structure of a carbonyl reductase, and MLCR is the first member of the SDR family to be solved in complex with NADPH (rather than NADH). Comparison of the MLCR ternary complex with three structures reported previously for enzymes of the SDR family (all crystallized as complexes with NADH) reveals a pair of basic residues (Lys17 and Arg39) making strong electrostatic interactions with the 2'-phosphate group of NADPH. This pair of residues is well conserved among the NADPH-preferring enzymes of the SDR family, but not among the NADH-preferring enzymes. In the latter, an aspartate side chain occupies the position of the two basic side chains. The aspartate residue, which would come into unacceptably close contact with the 2'-phosphate group of the adenosine moiety of NADPH, is replaced by a threonine or alanine in the primary sequences of NADPH-preferring enzymes of the SDR family.

CONCLUSIONS

The cofactor preferences exhibited by the enzymes of the SDR family are mainly determined by the electrostatic environment surrounding the 2'-hydroxyl (or phosphate) group of the adenosine ribose moiety of NADH (or NADPH). Thus, positively charged and negatively charged environments correlate with preference for NADPH and NADH respectively.

Crystal structure of rat liver dihydropteridine reductase

The structure of a binary complex of dihydropteridine reductase [DHPR; NAD(P)H:6,7-dihydropteridine oxidoreductase, EC 1.6.99.7] with its cofactor, NADH, has been solved and refined to a final R factor of 15.4% by using 2.3 A diffraction data. DHPR is an alpha/beta protein with a Rossmann-type dinucleotide fold for NADH binding. Insertion of an extra threonine residue in the human enzyme is associated with severe symptoms of a variant form of phenylketonuria and maps to a tightly linked sequence of secondary-structural elements near the dimer interface. Dimerization is mediated by a four-helix bundle motif (two helices from each protomer) having an unusual right-handed twist. DHPR is structurally and mechanistically distinct from dihydrofolate reductase, appearing to more closely resemble certain nicotinamide dinucleotide-requiring flavin-dependent enzymes, such as glutathione reductase.

The estimation of dihydropteridine reductase in human blood cells

Significant homology between dihydropteridine reductase (DHPR) from rat and human sources has been established by the ability of polyclonal antibodies raised to the rat-liver enzyme to detect the human protein in Western blots. The antibody also reacted with a single protein in bovine, dog and porcine kidney extracts, however, only trace reactivity was detected in rabbit. Quantitation of Western blots by soft laser densitometry showed that the response was proportional to total protein present in analyses of both pure rat-liver enzyme samples and crude extracts of rat and human liver. The DHPR contents of human blood cells were analysed by this method and the results compared to levels determined in enzymatic assays. Extracts of platelets and lymphocytes showed good correlation between these two methods, however, granulocytes exhibited high apparent enzyme activity but no DHPR protein detectable in blots. Erythrocyte extracts showed approximately 50% lower DHPR protein levels than predicted by activity measurements. These results are discussed in relation to the accuracy of detecting DHPR deficiencies in humans by enzymatic assay of whole blood samples.