Histidine reactions

Alkylation at pH 8.5 shows reduced rates of reaction at the histidine residues but significant substitution at lysine, particularly Lys 41 118). The histidine reactions show the same general stereospecificity as found at pH 5.5. The inactive Lys 41 derivatives (25, 26, and 27 of Table VI) show alkylation patterns of His 12 and 119 at pH 5.5 which are similar to those of RNase-A although with some differences in detail. When Lys 1 and 7 are acetylated in RNase-S the alkylation pattern with iodoacetic acid is not affected. When PIR is used the alkylation of His 119 is nearly abolished but that at His 12 is accelerated 163). The probable interaction of Asp 121 with His 119 may be important in the alkylation reactions observed in the native enzyme and the various lysine derivatives. In PIR this interaction has, of course, been removed. 

Potentiometric measurements have shown137 that the equilibrium constant for the D-glucose-L-histidine reaction falls as the temperature is raised from 20° to 50° the reaction is, therefore, exothermic. The heat of reaction for the 40-50° range was about twice that for the 20-40° range,... 

A base, formed by the bacterial degradation of histidine, and present in ergot and in many animal tissues, where it is liberated in response to injury and to antigen-antibody reactions. If injected it causes a condition of shock with dilatation of many blood vessels, loss of plasma from the capillaries to the tissues and a rapid fall in blood pressure. It is normally prepared from protein degradation products. 

Among the biochemical reactions that ammo acids undergo is decarboxylation to amines Decarboxylation of histidine for example gives histamine a powerful vasodila tor normally present m tissue and formed m excessive amounts under conditions of trau matic shock... 

The imidazole nng of the histidine side chain acts as a proton acceptor in certain enzyme catalyzed reactions Which is the more stable protonated form of the histidine residue A or Why" ... 

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 ). 

Fig. 2. Reaction of diphosphoglycerate (2,3-DPG) and deoxyhemoglobin. The molecule fits into the central cavity of hemoglobin and forms salt bridges with valine NA(I)p, histidines NA2(2)p, H2I(I43)p, and lysine EF6(82)p. A, E, and E correspond to specific hemoglobin hehces and NA is the sequence...

Imidazole-4-catbonitrile, 5-amino-synthesis, 5, 463 Imidazolecarboxamide, cyano-reactions, 5, 436 Imidazole-4-carboxamide Hofmann reaction, 5, 435 Imidazole-4-carboxamide, 5-amino-in histidine biosynthesis, 1, 90 reactions... 

The dinitrophenyl group has been used to protect the imidazole — NH group in histidines (45% yield)" by reaction with 2,4-dinitrofluorobenzene and potassium carbonate. Imidazole —NH groups, but not a-amino acid groups, are quantitatively regenerated by reaction with 2-mercaptoethanol (22°, pH 8, 1 h)." The 2,4-... 

The results supported the proposal of Glu-165 as the general base and suggested the novel possibility of neutral histidine acting as an acid, contrary to the expectation that His-95 was protonated [26,58]. The conclusion that the catalytic His-95 is neutral has been confinned by NMR spectroscopy [60]. The selection of neutral imidazole as the general acid catalyst has been discussed in terms of achieving a pX, balance with the weakly acidic intermediate. This avoids the thermodynamic trap that would result from a too stable enediol intermediate, produced by reaction with the more acidic imidazolium [58]. 

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

Among the biochemical reactions that anino acids undergo is decarboxylation to fflnines. Decar boxylation of histidine, for example, gives histamine, a powerful vasodilator nonnally present in tissue and fonned in excessive fflnounts under conditions of traumatic shock. 

FIGURE 19.24 A mechanism for the phosphoglycerate mutase reaction in rabbit muscle and in yeast. Zelda Rose of the Institute for Cancer Research in Philadelphia showed that the enzyme requires a small amount of 2,3-BPG to phosphorylate the histidine residue before the mechanism can proceed. Prior to her work, the role of the phosphohistidine in this mechanism was not understood. 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

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. 

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. 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

This intermediate expels a dtacylglycerol as leaving group in a nucleophilic acyl substitution reaction, giving an acyl enzyme. The dtacylglycerol is protonated by the histidine cation. 

The tetrahedral intermediate expels the serine as leaving group in a second nucleophilic acyl substitution reaction, yielding a free fatty acid. The serine accepts a proton from histidine, and the enzyme has now returned to its starting structure. 

Figure 29.2 MECHANISM Mechanism of action of lipase. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine, which react cooperatively to carry out two nucleophilic acyl substitution reactions. Individual steps are explained in the text.

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