Aldo-keto reductases in which the conserved catalytic histidine is substituted
Introduction
Two of the most prominent NAD(P)(H)-dependent oxidoreductases are the short-chain dehydrogenase reductases (SDRs) and the aldo-keto reductases (AKRs) [1], [2], [3], [4], [5]. These two enzyme superfamilies adopt different protein folds but share common endogenous substrates, e.g., steroid hormones, prostaglandins, and lipid aldehydes, as well as exogenous substrates such as xenobiotics, including aldehydes, ketones, and carbonyl containing drugs. Despite the differing structures of SDRs and AKRs, these enzyme superfamilies share common features of catalytic mechanism. The SDR active site contains a YXX(S)K motif, while the AKR active site contains conserved residues D50, Y55, K84, and H117 [4], [5], [6], [7], [8], [9], [10], [11]. Superposition of the active site residues of the SDR 3α(20β)-hydroxysteroid dehydrogenase (HSD) from Streptomyces hydrogenas[7] and rat 3α-hydroxysteroid dehydrogenase (AKR1C9) [11] shows that the general positions of the conserved tyrosine and lysine residues are generally similar relative to the si and re face of the nicotinamide ring of the cofactor, which donates either the 4-pro-S or 4-pro-R hydride in the SDR or AKR reactions, respectively (Fig. 1). It was also noted that the Ser in 3α(20β)-HSD and the Nɛ2 atom of His in AKR1C9 are both within hydrogen bonding distance of a water molecule that is displaced by the substrate carbonyl, suggesting that these residues play a facilitatory role in catalysis [7], [12]. It is apparent that the two enzyme superfamilies have convergently evolved to a common catalytic mechanism in which the tyrosine residue serves as a general acid–base [11]. In the SDRs the pKa of the Tyr is likely lowered by interaction with the nearby lysine residue, and the conserved Ser donates a hydrogen bond to the substrate carbonyl group (Fig. 2). In the AKRs, the Tyr acts a general base due to deprotonation by the lysine, but acts as a general acid by participating in a proton relay with the conserved His (Fig. 2) [13].
In the AKR superfamily, there are two subfamilies in which the conserved catalytic histidine residue is substituted [5]. In the AKR1D subfamily, which contains the steroid 5β-reductases, H117 is substituted by glutamic acid. In the AKR6A family, which contains the β-subunit Kvβ of the potassium channel Kv1, H117 is substituted by asparagine. Recently determined crystal structures of these AKRs have led to mechanistic proposals concerning the respective functions of these substituted residues [14]. These proposals are supported by comparisons to SDRs.
Section snippets
Comparison of the steroid double-bond reductases AKR1D1 and progesterone 5β-reductase
There are two forms of steroid hormone double-bond reductases that reduce Δ4-3-ketosteroids to the corresponding 5α- and 5β-reduced dihydrosteroids (Fig. 3). The steroid 5α-reductases are annotated as SRD5 genes and belong to neither the SDRs or the AKRs. In humans both type 1 and type 2 5α-reductases are found [15]. These enzymes generate steroid products containing a trans-A/B ring configuration. In contrast, the steroid 5β-reductases catalyze a reaction that is unique in steroid enzymology
Comparison of AKR1D1 and the Kvβ subunit of potassium channel Kv1
Voltage-gated potassium-selective channels are found in virtually all living organisms and form transmembrane pores. These pores are found in many cells types and tissues where they function to regulate electrical signaling and other physiological processes [22], [23], [24]. The pores are formed by the association of two subunits: the voltage-dependent potassium channel (Kv1) and a β-subunit (Kvβ) [25], [26], [27]. The crystal structure of the potassium channel associated with the β-subunit
Conflict of interest statement
The authors declare that they have no conflicts of interest.
Acknowledgements
Supported by grants R01-DK40715 to T.M.P and R01-GM56838 to D.W.C. We thank Dr. Ming Zhou for sharing the coordinates of the ternary complex Kvβ-NADPH-cortisone.
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