STRUCTURE OF BETAINE ALDEHYDE DEHYDROGENASE AT 2.1 &ARING RESOLUTION
Kenth Johansson1, Mustapha El-Ahmad1,2, Ramaswamy S.1, Lars Hjelmqvist2, Hans Jörnvall2 & Hans Eklund1
1Department of
Molecular Biology, Swedish University of Agricultural Sciences,
S-751 24 Uppsala, Sweden;
2Department of Medical Biochemistry and
Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden.
Keywords: Betaine aldehyde dehydrogenase, crystal structure, NAD+-binding
Aldehyde dehydrogenases (ALDHs) catalyse the irreversible oxidation of a broad range of aldehydes to the corresponding acids. Together with alcohol dehydrogenases, ALDHs are important components of cellular pathways that metabolize alcohols and aldehydes and they have been ascribed important functions in cellular detoxification and defense systems (Hempel et al., 1993, Jörnvall, 1994).
The human liver aldehyde dehydrogenase classes, ALDH1 (cytosolic) and ALDH2 (mitochondrial) have been studied extensively because of their involvement in the conversion of ethanol-derived acetaldehyde. A mutation in the oriental form of the mitochondrial enzyme results in low activity which is associated with alcohol intolerance. Recently, we purified and characterized the most abundant form of cod liver aldehyde dehydrogenase, betaine aldehyde dehydrogenase of the class 9 type. The betaine aldehyde dehydrogenase, ALDH9, has been thoroughly investigated in humans (Kurys et al., 1989; Chern & Pietruszko, 1995; Pietruszko et al., 1997). One postulated role for this enzyme is in the metabolism of putrescine to g-aminobutyric acid which is a principal neurotransmitter in the central nervous system. A second metabolic function of ALDH9 is related to its conversion of betaine aldehyde to betaine. Of the major enzyme classes, only ALDH9 catalyses this reaction at a significant level. Betaine can serve as a methyl donor for the biosynthesis of methionine and may also function as a defense against osmotic stress.
The first ALDH primary structure was established already 1984, of a class 1 enzyme (Hempel et al., 1984), while only recently three-dimensional structures of two ALDH classes were determined, those of the bovine class 2 ALDH (ALDH2) tetrameric enzyme (Steinmetz et al., 1997) and the rat class 3 ALDH (ALDH3) dimeric enzyme (Liu et al., 1997). In both cases the overall fold is very similar. However, a major difference appears to exist in the mode of coenzyme binding, where NAD+ binds deeper into the binding cleft of ALDH2 than of ALDH3. These differences has led to conflicting views of the catalytic mechanism. Because of these differences, the knowledge of additional structures of ALDHs is essential. We present here the three-dimensional structure of betaine ALDH (ALDH9) from cod liver in its apo and holo forms.
The three-dimensional structure of betaine aldehyde dehydrogenase was determined at 2.1 A resolution by the X-ray crystallographic method of molecular replacement. Like classes 2 and 3, the ALDH9 structure has three domains, one coenzyme binding domain, one catalytic domain and one oligomerization domain. Crystals grown in the presence or absence of NAD+ have very similar structures and no significant conformational change occurs upon coenzyme binding. The overall subunit structure is similar to that of the tetrameric bovine class 2 and dimeric rat class 3 ALDH, but the coenzyme binding with the nicotinamide in anti conformation, resembles that of class 2 rather than that of class 3.