Structural determinants and modulation of substrate specificity in phenylalanine-tyrosine ammonia-lyases - PubMed (original) (raw)

Structural determinants and modulation of substrate specificity in phenylalanine-tyrosine ammonia-lyases

Gordon V Louie et al. Chem Biol. 2006 Dec.

Abstract

Aromatic amino acid ammonia-lyases catalyze the deamination of L-His, L-Phe, and L-Tyr, yielding ammonia plus aryl acids bearing an alpha,beta-unsaturated propenoic acid. We report crystallographic analyses of unliganded Rhodobacter sphaeroides tyrosine ammonia-lyase (RsTAL) and RsTAL bound to p-coumarate and caffeate. His 89 of RsTAL forms a hydrogen bond with the p-hydroxyl moieties of coumarate and caffeate. His 89 is conserved in TALs but replaced in phenylalanine ammonia-lyases (PALs) and histidine ammonia-lyases (HALs). Substitution of His 89 by Phe, a characteristic residue of PALs, yields a mutant with a switch in kinetic preference from L-Tyr to L-Phe. Structures of the H89F mutant in complex with the PAL product, cinnamate, or the PAL-specific inhibitor, 2-aminoindan-2-phosphonate (AIP), support the role of position 89 as a specificity determinant in the family of aromatic amino acid ammonia-lyases and aminomutases responsible for beta-amino acid biosynthesis.

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Figures

Figure 1

Reactions Catalyzed by the Aromatic Amino Acid Ammonia-Lyases and the Related Aminomutases Tyrosine ammonia-lyase (TAL), phenylalanine ammonia-lyase (PAL), and histidine ammonia-lyase (HAL) catalyze the nonoxidative deamination of their respective amino acid substrates, yielding the corresponding α-β unsaturated aryl-acid product plus ammonia. The aminomutases catalyze the α-β migration of the amino group of the α-amino acid substrate. Labeling experiments [1, 15] have shown that the yellow β-proton (pro-S in L-Phe and L-Tyr, pro-R in L-His) of the substrate is stereospecifically abstracted. The dashed arrows emphasize the intermediacy of the lyase reaction in the overall reaction catalyzed by the aminomutases, which invoke Michael addition of ammonia to Cβ of the aryl-acid intermediate [, –16].

Figure 2

Three-Dimensional Structure of _Rs_TAL (A) Ribbon representations of the _Rs_TAL homotetramer, with the polypeptide chains of the individual monomers colored green (a), cyan (b), magenta (c), and yellow (d). The atoms of the four MIO cofactors are drawn as color-coded van der Waals spheres with red for oxygen, light gray for carbon, and blue for nitrogen. Orthogonal views from the top (left) and front (right) of the homotetramer are shown. The 222-point symmetry of the homotetramer is generated by three mutually orthogonal and intersecting 2-fold rotational axes, shown as gray lines. In each orientation, two of the axes are visible, with the third axis perpendicular to the page. (B) Ribbon representation of the _Rs_TAL monomer. The polypeptide chain is colored according to a gradient, with blue and red serving as extremes for the N- and C termini, respectively. The atoms of the MIO cofactor formed by the tripeptide segment Ala 149-Ser 150-Gly 151 are drawn as balls and sticks color-coded by atom type. The 2-fold axes that relate this monomer to the other monomers in the homotetramer are shown as gray lines. (C) Electron density and interactions of the active-site lid loops of _Rs_TAL complexed with coumarate shown as a stereo pair. The three-stranded β sheet is shown at the upper left. Three residues of the inner loop, Tyr 60, Phe 66, and Gly 67, encompass the bound coumarate product. Backbone hydrogen-bonding interactions of the lid loop are shown as magenta dashed lines; hydrogen-bonding interactions involving coumarate are represented as green dashed lines. The blue-colored contours envelop regions greater than 1.0σ in the final 2Fobs − Fcalc electron-density map calculated at 1.58 Å resolution. (D) Methylidene-imidazolone (MIO) cofactor. MIO and protein residues are shown as balls and sticks colored by atom type. Hydrogen-bonding interactions are represented as green dashed lines. An oxyanion hole is formed by the backbone amides of Leu 153 and Gly 204. The 149-150-151 numbering indicates the amino acid origin of the MIO cofactor. The blue-colored contours envelop regions greater than 3σ in the MIO-omit Fobs − Fcalc electron-density map. The inset shows the atom nomenclature of the native MIO cofactor, with the atom names colored according to atom type and numbered according to the originating residue within the 149–151 tripeptide (Ala 149 is 1, Ser 150 is 2 and Gly 151 is 3).

Figure 3

Active Site of _Rs_TAL (A) Partial amino acid (single letter codes) sequence alignment of _Rs_TAL with representative members of the aromatic amino acid ammonia-lyase family discussed in the text. Only regions that form the active site of the enzymes are shown. Numbering is according to _Rs_TAL. Yellow boxes highlight conserved catalytic and binding residues, while the green box highlights the specificity determining residues. (B) Electron density and interactions of the coumarate product bound in the active site of wild-type _Rs_TAL shown as a stereo pair. The coumarate, MIO cofactor, and protein side chains that line the active-site pocket are rendered as balls and sticks and colored according to atom type. Hydrogen-bonding interactions are shown as green dashed lines. The blue-colored contours envelop regions greater than 2.5σ in the initial Fobs − Fcalc electron-density map calculated at 1.58 Å resolution with phases derived from the unliganded model. The closest atom of coumarate to the MIO cofactor (labels colored green) is labeled Cβ and colored red. (C) Electron density and interactions of the caffeate product bound in the active site of wild-type _Rs_TAL. The phenyl ring of caffeate adopts primarily the conformation shown; a second, lower-occupancy conformation that differs only in the (inward) position of the meta-hydroxyl group is also observed. The blue-colored contours envelop regions greater than 2.5σ in the initial Fobs − Fcalc electron-density map calculated at 1.90 Å resolution with phases derived from the unliganded model. (D) Product binding pocket in _Rs_TAL. The depicted surface represents the area accessible to a probe sphere 1.4 Å in radius and the surface is color-coded according to the identity of the underlying protein atom (carbon is gray; nitrogen is blue; oxygen is red). The front portion of the _Rs_TAL tetramer has been cut away to reveal the internal cavity in the vicinity of the MIO cofactor. The coumarate molecule (shown in cyan) was excluded in the calculation of the molecular surface. The position of a caffeate molecule bound to _Rs_TAL is shown in yellow. MIO is labeled (green) as shown in the inset of Figure 2D.

Figure 4

Product Complexes of H89F _Rs_TAL (A) Electron density and interactions of the cinnamate product bound in the active site of H89F _Rs_TAL. For the Fobs − Fcalc electron-density map (1.9 Å resolution and shown contoured at 2.5σ), cinnamate and the side chain of Phe 89 were excluded from all calculations. (B) Electron density and interactions of coumarate bound to the H89F _Rs_TAL active site. For the Fobs − Fcalc electron-density map (2.0 Å resolution and shown contoured at 2.5σ), coumarate and the side chain of Phe 89 were excluded from all calculations. (C) Chemical structure of AIP. (D) Electron density and interactions of the PAL inhibitor AIP bound covalently to the MIO cofactor of H89F _Rs_TAL. For the Fobs − Fcalc electron-density map (1.75 Å resolution and shown contoured at 2.5σ), AIP, MIO, and the side chain of Phe 89 were excluded from all calculations. The residue labeled in red, Asn 203, is involved in a backbone conformational rearrangement that allows the Asn 203 side chain to engage the MIO cofactor upon AIP binding. (E) Comparison of the binding modes of cinnamate (yellow), coumarate (orange-tan), and AIP (cyan) with the H89F _Rs_TAL active site. Also shown is coumarate (magenta) bound to wild-type _Rs_TAL, with the hydrogen-bonding interactions between the coumarate product and wild-type _Rs_TAL represented as green dashed lines.

Figure 5

Active-Site Lid Loops and a Model for L-Tyr Binding to _Rs_TAL (A) Stereo comparison of the lid-loop conformations of _Rs_TAL (green), _Pc_PAL (blue), and _Pp_HAL (magenta). Two panels, top (_Rs_TAL versus _Pc_PAL) and bottom, (_Rs_TAL versus _Pp_HAL), are shown to simplify viewing. For each protein, the inner loops (_Rs_TAL residues 53–81, _Pc_PAL residues 103–129, and _Pp_HAL residues 46–74) are shown in darker shades, and the outer loops (_Rs_TAL residues 268–300, _Pc_PAL residues 325–351, and _Pp_HAL residues 260–280) are shown in lighter shades. The side chains of two invariant aromatic residues from the inner loop are shown (_Rs_TAL Tyr 60 and Phe 66, _Pc_PAL Tyr 110 and Phe 116, and _Pp_HAL Tyr 53 and Phe 59). (B) The _Rs_TAL homotetramer in the vicinity of the active-site pocket of monomer a is shown in ribbon representation. The polypeptide chains of the individual monomers are colored as in Figure 1A, with the active-site lid loops shaded darker (green, inner loop of monomer a; yellow, outer loop of monomer d). The MIO cofactor, bound coumarate, and protein residues that interact with the coumarate are drawn as balls and sticks and colored by atom type. (C) The L-Tyr substrate (magenta) was modeled with minimal modifications from the binding mode of the coumarate product shown in Figure 3B. (D) The L-Tyr substrate was modeled based upon the binding mode of the AIP inhibitor shown in Figure 4D, which places the α-amino group within covalent bonding distance of the Cβ2 methylidene carbon of the MIO cofactor (yellow dashed line). Note that a hydrogen bond between the L-Tyr-OH and the His 89-NE2 is preserved despite the shifted position of the L-Tyr substrate.

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Structural determinants and modulation of substrate specificity in phenylalanine-tyrosine ammonia-lyases - PubMed (original) (raw)