Hydrolases mechanism

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. 

Morisseau C, Hammock BD. Epoxide hydrolases mechanisms, inhibitor designs, and biological roles. Annu Rev Pharmacol Toxicol 2005 45 311-333. 

Serine Hydrolase Mechanism for the Acylation of Alcohols and Amines... 

Hydrolases possess a common feature for its mechanistic action. Their active site is built around three key amino acid residues, which are histidine (His), serine (Ser), and aspartic acid (Asp) or alternatively glutamic acid (Glu). This site, namely, the catalytic triad, forms a charge-relay network to polarize and activate the nucleophile, allowing the serine hydrolase mechanism to start [19]. This review covers the acylation of alcohols and amines using mainly esters so Figure 9.1 depicts the mechanism for these types of reactions is depicted. 

Some G proteins are slow GTP hydrolases with turnover numbers around two per minute, others such as Ras are only marginally catalytic. Kinetic experiments in solution have shown that in both cases the most likely mechanism... 

Proteases (proteinases, peptidases, or proteolytic enzymes) are enzymes that break peptide bonds between amino acids of proteins. The process is called peptide cleavage, a common mechanism of activation or inactivation of enzymes. They use a molecule of water for this, and are thus classified as hydrolases. 

SxxK (3-lactamases are uncoupled SxxK acyl transferases that work as (3-lactam antibiotic hydrolases. They represent a mechanism of defence of great efficiency. On good (3-lactam substrates, their catalytic centres can turn over 1000 times per second. 

Figure 3. Mitochondrial fatty acid oxidation. Long-chain fatty acids are converted to their CoA-esters as described in the text, and their fatty-acyl-groups transferred to CoA in the matrix by the concerted action of CPT 1, the acylcarnitine/carnitine exchange carrier and CPT (A) as described in the text. Medium-chain and short-chain fatty acids (Cg or less) diffuse directly into the matrix where they are converted to their acyl-CoA esters by a acyl-CoA synthase. The mechanism of p-oxidation is shown below (B). Each cycle of P-oxidation removes -CH2-CH2- as an acetyl unit until the fatty acids are completely converted to acetyl-CoA. The enzymes catalyzing each stage of P-oxidation have different but overlapping specificities. In muscle mitochondria, most acetyl-CoA is oxidized to CO2 and H2O by the citrate cycle (Figure 4) some is converted to acylcamitine by carnitine acetyltransferase (associated with the inner face of the inner membrane) and exported from the matrix. Some acetyl-CoA (if in excess) is hydrolyzed to acetate and CoASH by acetyl-CoA hydrolase in the matrix. Enzymes ...

In this article are discussed the results of those studies which have become available over the past 15 years and which permit some generalizations on the catalytic mechanism of glycoside hydrolases from widely differing sources and with different sugar and aglycon specificities. It will be seen that, with few exceptions, the data support a mechanism almost identical to that proposed by Phillips and his group for lysozyme. ... 

The results confirm that RG-hydrolase is a true rhamnogalacturonase, as it splits an alternating RG chain, and furthermore that it cleaves between GalA and Rha in the main chain by a mechanism of hydrolysis (Schols et al., 1990a). Similarly, the products from the RGO s with higher DP s were characterised. 

The lack of glycosyl transfer reaction is the class of pectinolytic hydrolases is in agreement with the observed inversion of the anomeric configuration of the newly formed reducing ends of the products. All three polygalacturonases studied here utilize the single displacement mechanism of hydrolysis. 

Rink R, M Eennema, M Smids, U Dehmel, DB Janssen (1997) Primary structure and catalytic mechanism of the epoxide hydrolase from Agrobacterium radiobacter ADI. J Biol Chem 272 14650-14657. 

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