A-Histidine

The reaction between esterase and phosphorus inhibitor (109) is bimolecular, of the weU-known S 2 type, and represents the attack of a nucleophilic serine hydroxyl with a neighboring imida2ole ring of a histidine residue at the active site, on the electrophilic phosphorus atom, and mimics the normal three-step reaction that takes place between enzyme and substrate (reaction ). 

Alpha helices D and E from the L and M subunits (Figure 12.14) form the core of the membrane-spanning part of the complex. These four helices are tightly packed against each other in a way quite similar to the four-helix bundle motif in water-soluble proteins. Each of these four helices provides a histidine side chain as ligand to the Ee atom, which is located between the helices close to the cytoplasm. The role of the Ee atom is probably to... 

Figure 12.18 Ribbon diagram showing the a (red) and the P (blue) chains of the light-harvesting complex LH2. Each chain forms one transmembrane a helix, which contains a histidine residue that binds to the Mg atom of one bacteriochlorophyll molecule. (Adapted from G. McDermott et al.. Nature 374 517-521, 1995.)...

FIGURE 14.11 The pH activity profiles of four different enzymes. Trypsin, an intestinal protease, has a slightly alkaline pH optimnm, whereas pepsin, a gastric protease, acts in the acidic confines of the stomach and has a pH optimmn near 2. Papain, a protease found in papaya, is relatively insensitive to pHs between 4 and 8. Cholinesterase activity is pH-sensitive below pH 7 but not between pH 7 and 10. The cholinesterase pH activity profile suggests that an ionizable group with a pK near 6 is essential to its activity. Might it be a histidine residue within the active site ... 

FIGURE 20.15 The covalent bond between FAD and succinate dehydrogenase involves the C-8a methylene group of FAD and the N-3 of a histidine residue on the enzyme. 

Complex II is perhaps better known by its other name—succinate dehydrogenase, the only TCA cycle enzyme that is an integral membrane protein in the inner mitochondrial membrane. This enzyme has a mass of approximately 100 to 140 kD and is composed of four subunits two Fe-S proteins of masses 70 kD and 27 kD, and two other peptides of masses 15 kD and 13 kD. Also known as flavoprotein 2 (FP2), it contains an FAD covalently bound to a histidine residue (see Figure 20.15), and three Fe-S centers a 4Fe-4S cluster, a 3Fe-4S cluster, and a 2Fe-2S cluster. When succinate is converted to fumarate in the TCA cycle, concomitant reduction of bound FAD to FADHg occurs in succinate dehydrogenase. This FADHg transfers its electrons immediately to Fe-S centers, which pass them on to UQ. Electron flow from succinate to UQ,... 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

The side-chain carboxylate group of an aspartic acid acts as a base and removes an acidic a proton from acetyl CoA, while the N-H group on the side chain of a histidine acts as an acid and donates a proton to the car bonyl oxygen, giving an enol. 

A histidine deprotonates the acetyl-CoA enol, which adds to the ketone carbonyl group of oxaloacetate in an aldol-like reaction. Simultaneously, an acid N-H proton of another histidine protonates the carbonyl oxygen, producing (S)-citryl CoA. 

Q The enzyme active site contains an aspartic acid, a histidine, and a serine. First, histidine acts as a base to deprotonate the -OH group of serine, with the negatively charged carboxylate of aspartic acid stabilizing the nearby histidine cation that results. Serine then adds to the carbonyl group of the triacylglycerol, yielding a tetrahedral intermediate. 

Step 3 of Figure 29.3 Alcohol Oxidation The /3-hydroxyacyl CoA from step 2 is oxidized to a /3-ketoacyl CoA in a reaction catalyzed by one of a family of L-3-hydroxyacyl-CoA dehydrogenases, which differ in substrate specificity according to the chain length of the acyl group. As in the oxidation of sn-glycerol 3-phosphate to dihydroxyacetone phosphate mentioned at the end of Section 29.2, this alcohol oxidation requires NAD+ as a coenzyme and yields reduced NADH/H+ as by-product. Deprotonation of the hydroxyl group is carried out by a histidine residue at the active site. 

Interestingly, however, the mechanisms of the two phosphate hydrolysis reactions in steps 9 and 11 are not the same. In step 9, water is the nucleophile, but in the glucose 6-phosphate reaction of step 11, a histidine residue on the enzyme attacks phosphorus, giving a phosphoryl enzyme intermediate that subsequently reacts with water. 

The mechanism for the lipase-catalyzed reaction of an acid derivative with a nucleophile (alcohol, amine, or thiol) is known as a serine hydrolase mechanism (Scheme 7.2). The active site of the enzyme is constituted by a catalytic triad (serine, aspartic, and histidine residues). The serine residue accepts the acyl group of the ester, leading to an acyl-enzyme activated intermediate. This acyl-enzyme intermediate reacts with the nucleophile, an amine or ammonia in this case, to yield the final amide product and leading to the free biocatalyst, which can enter again into the catalytic cycle. A histidine residue, activated by an aspartate side chain, is responsible for the proton transference necessary for the catalysis. Another important factor is that the oxyanion hole, formed by different residues, is able to stabilize the negatively charged oxygen present in both the transition state and the tetrahedral intermediate. 

In vanadium-dependent haloperoxidases, the metal center is coordinated to the imidazole system of a histidine residue, which is similarly responsible for creating hypochlorite or hypobromite as electrophilic halogenating species [274]. Remarkably, a representative of this enzyme class is capable of performing stereoselective incorporation of halides, as has been reported for the conversion of nerolidol to various snyderols. The overall reaction commences through a bromonium intermediate, which cyclizes in an intramolecular process the resulting carbocation can ultimately be trapped upon elimination to three snyderols (Scheme 9.37) [275]. 

Another example of noncysteinyl coordination of a [4Fe-4S] center was provided by the resolution of the X-ray structure of D. gigas hy-drogenase, which revealed that one iron atom of the distal [4Fe-4S] center was ligated by the atom of a histidine residue (164). Unfortunately, the EPR signature of this center could not be determined because of intercenter spin—spin interactions with other iron—sulfur centers (95). 

Fig. 15 Amino acid sequences of artificial extracellular matrix (aECM) proteins. Each protein contains a TV tag, a histidine tag, a cleavage site, and elastin-like domains with lysine residues for crosslinking. The RGD cell-binding domain is found in aECM 1, whereas aECM 3 contains the CS5 cell-binding domain. aECM 2 and aECM 4 are the negative controls with scrambled binding domains for aECM 1 and aECM 3, respectively. Reprinted from [121] with permission from American Chemical Society, copyright 2004...

A different type of reactive bromo compound having a moderate resemblance to hexoses is represented by the bromoconduritols B (40) and F (41), named after the respective parent tetrahydroxycyclohexene. Even thou their hydroxylation pattern resembles that of D-glucose, only a few examples of D-glucosidase inhibition have been reported. The first was a-D-glucosi-dase from yeast, which is inhibited by bromoconduritol B (formerly called bromoconduritol A), having ki(max)/Kj 69,000 M" min", by alkylation of a histidine residue at the active site. 

Each heme unit in myoglobin and hemoglobin contains one ion bound to four nitrogen donor atoms in a square planar arrangement. This leaves the metal with two axial coordination sites to bind other ligands. One of these sites is bound to a histidine side chain that holds the heme in the pocket of the protein. The other axial position is where reversible binding of molecular oxygen takes place. 

The structure of the enzymatic site of superoxide dismutase. The proximity, with a histidine side chain (color screened) acting as a bridge between the metals. 

Cytochromes. A cytochrome is a protein containing a heme with an iron cation bonded to four donor nitrogen atoms in a square planar array. Figure 20-29a shows the structure of cytochrome c, in which a histidine nitrogen atom and a cysteine sulfur atom occupy the fifth and sixth coordination sites of the octahedral iron center. 

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