The logic synthesis of ultra-high-speed FSMs is presented. The state assignment is based on a well-known method that uses output vectors. This technique is adjusted to include elements of two-level ...minimization and takes into account the limited number of terms contained in the programmable-AND/fixed-OR logic cell. The state assignment is based on a special form of the binary decision tree. The second phase of the FSM design is logic optimization. The optimization method is based on tristate buffers, thus making possible a one-logic-level FSM structure. The key point is to search partition variables that control the tristate buffers. This technique can also be applied to combinational circuits or the output block of FSMs only.
Algorithms for state assignment and optimization are presented and richly illustrated by examples. The method is dedicated to using specific features of complex programmable logic devices. Experimental results prove its effectiveness (e.g., the implementation of the the 16-bit counter requires 136 logic cells and one-logic-cell level instead of 213 cells and four levels). The optimization method using tristate buffers and a state assignment binary decision tree can be directly applied to FPGA-dedicated logic synthesis.
YwhB, a 4-oxalocrotonate tautomerase (4-OT) homologue in Bacillus subtilis, has no known biological role, and the gene has no apparent genomic context. The kinetic and stereochemical properties of ...YwhB have been examined using available enol and dienol compounds. The kinetic analysis shows that YwhB has a relatively nonspecific 1,3- and 1,5-keto−enol tautomerase activity, with the former activity prevailing. Replacement of Pro-1 or Arg-11 with an alanine significantly reduces or abolishes these activities, implicating both residues as critical ones for the activities. In D2O, ketonization of two monoacid substrates (2-hydroxy-2,4-pentadienoate and phenylenolpyruvate) produces a mixture of stereoisomers {2-keto-3-2H-4-pentenoate and 3-2H-phenylpyruvate}, where the (3R)-isomers predominate. Ketonization of 2-hydroxy-2,4-hexadienedioate, a diacid, in D2O affords mostly the opposite enantiomer, (3S)-2-oxo-3-2H-4-hexenedioate. The mono- and diacids apparently bind in different orientations in the active site of YwhB, but the highly stereoselective nature of the YwhB reaction using a diacid suggests that the biological substrate for YwhB may be a diacid. Moreover, of the three dienols examined, 1,3- and 1,5-keto−enol tautomerization reactions are only observed for 2-hydroxy-2,4-hexadienedioate, indicating that the C-3 and C-5 positions are accessible for protonation in this compound. Incubation of 4-OT with 2-hydroxy-2,4-hexadienedioate in D2O results in a racemic mixture of 2-oxo-3-2H-4-hexenedioate, suggesting that 4-OT may not catalyze a 1,3-keto−enol tautomerization reaction using this dienol. It has previously been shown that 4-OT catalyzes the near stereospecific conversion of 2-hydroxy-2,4-hexadienedioate to (5S)-5-2H-2-oxo-3-hexenedioate in D2O. Taken together, these observations suggest that 4-OT might function as a 1,5-keto−enol tautomerase using 2-hydroxy-2,4-hexadienedioate.
Macrophage migration inhibitory factor (MIF) is a cytokine that was first described as an inhibitor of the random migration of monocytes and macrophages and has since been proposed to have a number ...of immune and catalytic functions. One of the functions assigned to MIF is that of a tautomerase that interconverts the enol and keto forms of phenylpyruvate and (p-hydroxyphenyl)pyruvate and converts d-dopachrome, a stereoisomer of naturally occurring l-dopachrome, to 5,6-dihydroxyindole-2-carboxylic acid. The physiological significance of the MIF enzymatic activity is unclear. The three-dimensional structure of MIF is strikingly similar to that of two microbial enzymes (4-oxalocrotonate tautomerase and 5-carboxymethyl-2-hydroxymuconate isomerase) that otherwise share little sequence identity with MIF. MIF and these two enzymes have an invariant N-terminal proline that serves as a catalytic base. Here we report a new biological function for MIF, as an inhibitor of monocyte chemoattractant protein 1- (MCP-1-) induced chemotaxis of human peripheral blood monocytes. We find that MIF inhibition of chemotaxis does not occur at the level of the CC chemokine receptor for MCP-1, CCR2, since MIF does not alter the binding of 125I-MCP-1 to monocytes. The role of MIF enzymatic activity in inhibition of monocyte chemotaxis and random migration was studied with two MIF mutants in which the N-terminal proline was replaced with either a serine or a phenylalanine. Both mutants remain capable of inhibiting monocyte chemotaxis and random migration despite significantly reduced or no phenylpyruvate tautomerase activity. These data suggest that this enzymatic activity of MIF does not play a role in its migration inhibiting properties.
MAP KAP kinase 2 (MK2), a Ser/Thr kinase, plays a crucial role in the inflammatory process. We have determined the crystal structures of a catalytically active C-terminal deletion form of human MK2, ...residues 41–364, in complex with staurosporine at 2.7 Å and with ADP at 3.2 Å, revealing overall structural similarity with other Ser/Thr kinases. Kinetic analysis reveals that the
K
m
for ATP is very similar for MK2 41–364 and p38-activated MK2 41–400. Conversely, the catalytic rate and binding for peptide substrate are dramatically reduced in MK2 41–364. However, phosphorylation of MK2 41–364 by p38 restores the V
max and
K
m
for peptide substrate to values comparable to those seen in p38-activated MK2 41–400, suggesting a mechanism for regulation of enzyme activity.
The nadD gene, encoding the enzyme nicotinic acid mononucleotide (NaMN) adenylyltransferase (AT), is essential for the synthesis of NAD and subsequent viability of the cell. The nadD gene in Bacillus ...subtilis (yqeJ) was identified by sequence homology with other bacterial nadD genes and by biochemical characterization of the gene product. NaMN AT catalyzes the reversible adenylation of both NaMN and the nicotinamide mononucleotide (NMN) but shows specificity for the nicotinate. In contrast to other known NMN ATs, biophysical characterizations reveal it to be a dimer. The NaMN AT crystal structure was determined for both the apo enzyme and product-bound form, to 2.1 and 3.2 A, respectively. The structures reveal a "functional" dimer conserved in both crystal forms and a monomer fold common to members of the nucleotidyl-transferase alpha/beta phosphodiesterase superfamily. A structural comparison with family members suggests a new conserved motif (SXXXX(R/K)) at the N terminus of an alpha-helix, which is not part of the shared fold. Interactions of the nicotinic acid with backbone atoms indicate the structural basis for specificity.
Macrophage migration inhibitory factor (MIF) exhibits dual activities. It acts as an immunoregulatory protein as well as a phenylpyruvate tautomerase. To understand better the relationship between ...these two activities and to elucidate the structural basis for the enzymatic activity, a crystal structure of a complex between murine MIF and (E)-2-fluoro-p-hydroxycinnamate, a competitive inhibitor of the tautomerase activity, has been determined to 1.8 Å resolution. The structure is nearly superimposable on that of the free protein indicating that the presence of the inhibitor does not result in any major structural changes. The inhibitor also confirms the location of the active site in a hydrophobic cavity containing the amino-terminal proline. Within this cavity, the inhibitor interacts with residues from adjacent subunits. At the back of the cavity, the side-chain carbonyl oxygen of Asn-97‘ interacts with the phenolic hydroxyl group of the inhibitor while at the mouth of the cavity the ammonium group of Lys-32 interacts with a carboxylate oxygen. The other carboxylate oxygen of the inhibitor interacts with Pro-1. The hydroxyl group of Tyr-95‘ interacts weakly with the fluoro group on the inhibitor. The hydrophobic side chains of five active-site residues (Met-2, Ile-64, Met-101, Val-106, and Phe-113) and the phenyl moiety of Tyr-95‘ are responsible for the binding of the phenyl group. Further insight into the enzymatic activity of MIF was obtained by carrying out kinetic studies using the enol isomers of phenylpyruvate and (p-hydroxyphenyl)pyruvate. The results demonstrate that MIF processes the enol isomers more efficiently than the keto isomers primarily because of a decrease in K m. On the basis of these results, a mechanism is proposed for the MIF-catalyzed tautomerization reaction.
The electrostatic surface potentials of HpDmpI, 4-oxalocrotonate tautomerase (4-OT), and AfDmpI, respectively, from left to right. HpDmpI lacks electropositive character at one end of the proposed ...active site. 4-OT shows strong electropositive character at each end of the active site. AfDmpI lacks strong positive or negative electrostatic character.
The tautomerase superfamily consists of structurally homologous proteins that are characterized by a β-α-β fold and a catalytic amino-terminal proline. 4-Oxalocrotonate tautomerase (4-OT) family members have been identified and categorized into five subfamilies on the basis of multiple sequence alignments and the conservation of key catalytic and structural residues. Representative members from two subfamilies have been cloned, expressed, purified, and subjected to kinetic and structural characterization. The crystal structure of DmpI from
Helicobacter pylori (HpDmpI), a 4-OT homolog in subfamily 3, has been determined to high resolution (1.8
Å and 2.1
Å) in two different space groups. HpDmpI is a homohexamer with an active site cavity that includes Pro-1, but lacks the equivalent of Arg-11 and Arg-39 found in 4-OT. Instead, the side chain of Lys-36 replaces that of Arg-11 in a manner similar to that observed in the trimeric macrophage migration inhibitory factor (MIF), which is the title protein of another family in the superfamily. The electrostatic surface of the active site is also quite different and suggests that HpDmpI might prefer small, monoacid substrates. A kinetic analysis of the enzyme is consistent with the structural analysis, but a biological role for the enzyme remains elusive. The crystal structure of DmpI from
Archaeoglobus fulgidus (AfDmpI
), a 4-OT homolog in subfamily-4, has been determined to 2.4
Å resolution. AfDmpI is also a homohexamer, with a proposed active site cavity that includes Pro-1, but lacks any other residues that are readily identified as catalytic ones related to 4-OT activity. Indeed, the electrostatic potential of the active site differs significantly in that it is mostly neutral, in contrast to the usual electropositive features found in other 4-OT family members, suggesting that AfDmpI might accommodate hydrophobic substrates. A kinetic analysis has been carried out, but does not provide any clues about the type of reaction the enzyme might catalyze.
The crystal structure of 4-oxalocrotonate tautomerase (4-OT) inactivated by the active site-directed irreversible inhibitor 2-oxo-3-pentynoate (2-OP) has been determined to 2.4 Å resolution. The ...structure of the enzyme covalently modified at Pro-1 by the resulting 2-oxo-3-pentenoate adduct is nearly superimposable on that of the free enzyme and confirms that the active site is located in a hydrophobic region surrounding Pro-1. Both structures can be described as a trimer of dimers where each dimer consists of a four-stranded β-sheet with two antiparallel α-helices on one side. Examination of the structure also reveals noncovalent interactions between the adduct and two residues in the active site. The ε and η nitrogens of the guanidinium side chain of Arg-39‘ ‘ from a neighboring dimer interact respectively with the C-2 carbonyl oxygen and one C-1 carboxylate oxygen of the adduct while the side chain of Arg-61‘ from the same dimer as the modified Pro-1 interacts with the C-1 carboxylate group in a bidentate fashion. An additional interaction to the 2-oxo group of the adduct is provided by one of the two ordered water molecules within the active site region. These interactions coupled with the observation that 2-oxo-3-butynoate is a more potent irreversible inhibitor of 4-oxalocrotonate tautomerase than is 2-OP suggest that Arg-39‘ ‘ and the ordered water molecule polarize the carbonyl group of 2-OP which facilitates a Michael reaction between Pro-1 and the acetylene compound. On the basis of the crystal structure, a mechanism for the enzyme-catalyzed reaction is proposed.
Three arginine residues (Arg-11, Arg-39, Arg-61) are found at the active site of 4-oxalocrotonate tautomerase in the X-ray structure of the affinity-labeled enzyme Taylor, A. B., Czerwinski, R. M., ...Johnson, R. M., Jr., Whitman, C. P., and Hackert, M. L. (1998) Biochemistry 37, 14692−14700. The catalytic roles of these arginines were examined by mutagenesis, kinetic, and heteronuclear NMR studies. With a 1,6-dicarboxylate substrate (2-hydroxymuconate), the R61A mutation showed no kinetic effects, while the R11A mutation decreased k cat 88-fold and increased K m 8.6-fold, suggesting both binding and catalytic roles for Arg-11. With a 1-monocarboxylate substrate (2-hydroxy-2,4-pentadienoate), no kinetic effects of the R11A mutation were found, indicating that Arg-11 interacts with the 6-carboxylate of the substrate. The stereoselectivity of the R11A-catalyzed protonation at C-5 of the dicarboxylate substrate decreased, while the stereoselectivity of protonation at C-3 of the monocarboxylate substrate increased in comparison with wild-type 4-OT, indicating the importance of Arg-11 in properly orienting the dicarboxylate substrate by interacting with the charged 6-carboxylate group. With 2-hydroxymuconate, the R39A and R39Q mutations decreased k cat by 125- and 389-fold and increased K m by 1.5- and 2.6-fold, respectively, suggesting a largely catalytic role for Arg-39. The activity of the R11A/R39A double mutant was at least 104-fold lower than that of the wild-type enzyme, indicating approximate additivity of the effects of the two arginine mutants on k cat. For both R11A and R39Q, 2D 1H−15N HSQC and 3D 1H−15N NOESY−HSQC spectra showed chemical shift changes mainly near the mutated residues, indicating otherwise intact protein structures. The changes in the R39Q mutant were mainly in the β-hairpin from residues 50 to 57 which covers the active site. HSQC titration of R11A with the substrate analogue cis,cis-muconate yielded a K d of 22 mM, 37-fold greater than the K d found with wild-type 4-OT (0.6 mM). With the R39Q mutant, cis,cis-muconate showed negative cooperativity in active site binding with two K d values, 3.5 and 29 mM. This observation together with the low K m of 2-hydroxymuconate (0.47 mM) suggests that only the tight binding sites function catalytically in the R39Q mutant. The 15Nε resonances of all six Arg residues of 4-OT were assigned, and the assignments of Arg-11, -39, and -61 were confirmed by mutagenesis. The binding of cis,cis-muconate to wild-type 4-OT upshifts Arg-11 Nε (by 0.05 ppm) and downshifts Arg-39 Nε (by 1.19 ppm), indicating differing electronic delocalizations in the guanidinium groups. A mechanism is proposed in which Arg-11 interacts with the 6-carboxylate of the substrate to facilitate both substrate binding and catalysis and Arg-39 interacts with the 1-carboxylate and the 2-keto group of the substrate to promote carbonyl polarization and catalysis, while Pro-1 transfers protons from C-3 to C-5. This mechanism, together with the effects of mutations of catalytic residues on k cat, provides a quantitative explanation of the 107-fold catalytic power of 4-OT. Despite its presence in the active site in the crystal structure of the affinity-labeled enzyme, Arg-61 does not play a significant role in either substrate binding or catalysis.
The tautomerase superfamily consists of three major families represented by 4-oxalocrotonate tautomerase (4-OT), 5-(carboxymethyl)-2-hydroxymuconate isomerase (CHMI), and macrophage migration ...inhibitory factor (MIF). The members of this superfamily are structurally homologous proteins constructed from a simple β−α−β fold that share a key mechanistic feature; they use an amino-terminal proline, which has an unusually low pK a, as the general base in a keto−enol tautomerization. Several new members of the 4-OT family have now been identified using PSI-BLAST and categorized into five subfamilies on the basis of multiple-sequence alignments and the conservation of key catalytic and structural residues. The members of subfamily 5, which includes a hypothetical protein designated YdcE from Escherichia coli, are predicted not to form hexamers. The crystal structure of YdcE has been determined to 1.35 Å resolution and confirms that it is a dimer. In addition, YdcE complexed with (E)-2-fluoro-p-hydroxycinnamate, identified as a potent competitive inhibitor of this enzyme, as well as N-(2-hydroxyethyl)piperazine-N‘-2-ethanesulfonic acid (HEPES) and benzoate are also presented. These latter crystal structures reveal the location of the active site and suggest a mechanism for the observed YdcE-catalyzed tautomerization reaction. The dimeric arrangement of YdcE represents a new structure in the 4-OT family and demonstrates structural diversity within the 4-OT family not previously reported.
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