Histidine constitution

This model clearly shows that the catalytic machinery involves a dyad of histidine and aspartate together with the oxyanion hole. Hence, it does not involve serine, which is the key amino acid in the hydrolytic activity of lipases, and, together with aspartate and histidine, constitutes the active site catalytic triad. This has been confirmed by constructing a mutant in which serine was replaced with alanine (Serl05Ala), and finding that it catalyzes the Michael additions even more efficiently than the wild-type enzyme (an example of induced catalytic promiscuity ) [105]. 

Histidine Constitutes a Family of One Nonprotein Amino Acids Are Derived from Protein Amino Acids... 

AChE was first described in the late 1930s. Subsequent work with different substrates and inhibitors led to a conceptualization of the enzyme s active center involving two binding sites for ACh an ionic site (most likely the gam/mt-carboxylate anion of a glutamate residue) to which the cationic ACh head presumably is attracted and a serine residue, which, in cooperation with the imidazole ring of a histidine, constitutes an esteratic site. Here, the serine s hydroxyl group, as the primary nucleophile, initiates hydrolysis (Fig. 8-7). The distance between the two sites has been variously reported to be as little as 2.5-7 A. (The superficial similarity with the cholinergic receptor ends with the realization... 

There is some doubt as to whether histidine, in fact, is required in the therapy of renal failure but at the present time the evidence does not appear overwhelming. Since the question has been previously raised as to whether histidine constituted an essential amino acid in children (Stifel and Herman, 1972), it may therefore be that the L-essential amino acid mixture in a therapy of renal failure in children should be different than that of the adult. 

In order to further verify the involvement of catalytic amino acid residues in enzyme-catalyzed ring-opening polymerization, catalytic amino acids of the catalytic domain of the PHB depolymerase were replaced and evaluated for their polymerization activities. PHB depolymerases have structures that consist of a catalytic domain, a putative linker region, and a substrate binding domain (SBD). Three strictly conserved amino acids, serine, aspartate, and histidine, constitute the catalytic triad at the active center of the catalytic domain. The conserved serine is part of the so-called lipase-box... 

It is usually assumed that the histidine operon is transcribed into one molecule of messenger RNA, which is then transcribed by the polyribosome system from one end to the other. The histidine operon and corresponding mRNA consist of about 13,000 nucleotides. The possibility that such a polycistronic histidine messenger RNA exists was demonstrated by the work of Martin (1963). The RNA fraction, labeling of which took place differently in histidine-constitutive and histidine-deficient strains, had a sedimentation... 

In other words, the tautomerism of histidines constitutes a molecular environmental probe. Moreover, model studies for the interior of proteins should not be made using water but wet polar organic liquids as solvents. 

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. 

The constitution of all the amino acids except diaminotrioxy-dodecanic acid is thus known, and with the exception of histidine they have all been synthesised. The separation of the synthetical compound into its optical antipodes has still to be effected in the case of several of the amino acids. 

Based on the properties of the side chains, the 20 amino acids can be put into six general classes. The first class contains amino acids whose side chains are aliphatic, and is usually considered to include glycine, alanine, valine, leucine, and isoleucine. The second class is composed of the amino acids with polar, nonionic side chains, and includes serine, threonine, cysteine, and methionine. The cyclic amino acid proline (actually, an imino acid) constitutes a third class by itself. The fourth class contains amino acids with aromatic side chains tyrosine, phenylalanine, and tryptophan. The fifth class has basic groups on the side chains and is made up of the three amino acids lysine, arginine, and histidine. The sixth class is composed of the acidic amino acids and their amides aspartate and asparagine, and glutamate and glutamine. 

Figure 3-22 Stereoscopic view of a section of the structure of cutinase from the fungus Fusarium solani determined to a resolution of 0.10 nm. The three amino acid residues shown are serine 120 (top), histidine 188, and aspartate 175 (lower left). The structure is presented as a contour map with a "wire mesh" drawn at a "cutoff" level of density equal to 1 a above the average, where a is the root mean square density of the entire map. The side chains of these three residues constitute the "catalytic triad" in the active site of this enzyme (see Chapter 12). At this resolution more than one conformation of a group may often be seen. For example, the gamma oxygen (OG) of S120 is seen in two positions, the major one being toward His 188. When the map is drawn with a lower contour level the N-H proton on His 188 that is hydrogen bonded to Asp 175 can also be seen.410 Courtesy of Christian Cambillau.

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