Proteins acid-catalyzed hydrolysis

For acid-catalyzed hydrolysis of methyl glucosides53 the kinetic isotope effect observed for the oxygen of the leaving group was /c,6()//c18() = 1.024-1.026. Observation of similar effects for enzymes supports the participation of an acidic group of the protein (Glu 35 of lysozyme) in catalysis but does not eliminate the possibility of concerted involvement of a nucleophilic group, e.g., Asp 52 in lysozyme.81 82... 

The amide bonds in peptides and proteins can be hydrolyzed in strong acid or base Treatment of a peptide or protein under either of these conditions yields a mixture of the constituent amino acids. Neither acid- nor base-catalyzed hydrolysis of a protein leads to ideal results because both tend to destroy some constituent ammo acids. Acid-catalyzed hydrolysis destroys tryptophan and cysteine, causes some loss of serine and threonine, and converts asparagine and glutamine to aspartic acid and glutamic acid, respectively. Base-catalyzed hydrolysis leads to destruction of serine, threonine, cysteine, and cystine and also results in racemization of the free amino acids. Because acid-catalyzed hydrolysis is less destructive, it is often the method of choice. The hydrolysis procedure consists of dissolving the protein sample in aqueous acid, usually 6 M HC1, and heating the solution in a sealed, evacuated vial at 100°C for 12 to 24 hours. 

Acid hydrolysis of peptides yields the amino acids from which the peptide was assembled and a few other fragments arising from postassembly modifications. This is simply the acid-catalyzed hydrolysis of the peptide, that is, amide (Chapter 9) bonds. Hundreds, and in some cases thousands, of copies of amino adds are obtained on complete hydrolysis (sometimes called digestion ) of many peptides (proteins), and although any given one might be focused upon as exemplary, only one, a-chymotrypsin, because it has been studied so extensively, is used here. 

Nucleophilic attack by oxygen on the methylene group, resulting in departure of the weakly basic leaving group, forms a five-membered ring (Section 9.2). Acid-catalyzed hydrolysis of the imine cleaves the protein (Section 17.10). 

Step 1. Acid-catalyzed hydrolysis. We recall that amides, and therefore peptide bonds, ate quite stable and the fairly stringent conditions are required for this reaction. The protein is hydrolyzed in... 

Partial -+hydrolysis of collagen (- proteins)- containing animal products (skin, white connective tissues and bones) leads to g. Acid-catalyzed hydrolysis yields type A g., while alkali treatment gives B g. Edible g., used in pharmaceutical, food and photographic applications is made from carefully selected raw materials, while technical g. (- glue, animal) is similar in structure but starts from waste protein material. 

A variety of hydrolases catalyze the hydrolysis of acetylsalicylic acid. In humans, high activities have been seen with membrane-bound and cytosolic carboxylesterases (EC 3.1.1.1), plasma cholinesterase (EC 3.1.1.8), and red blood cell arylesterases (EC 3.1.1.2), whereas nonenzymatic hydrolysis appears to contribute to a small percentage of the total salicylic acid formed [76a] [82], A solution of serum albumin also displayed weak hydrolytic activity toward the drug, but, under the conditions of the study, binding to serum albumin decreased chemical hydrolysis at 37° and pH 7.4 from tm 12 1 h when unbound to 27 3 h for the fully bound drug [83], In contrast, binding to serum albumin increased by >50% the rate of carboxylesterase-catalyzed hydrolysis, as seen in buffers containing the hydrolase with or without albumin. It has been postulated that either bound acetylsalicylic acid is more susceptible to enzyme hydrolysis, or the protein directly activates the enzyme. 

Enzymatic Hydrolysis Reactions of Esters. Xenobiotic compounds containing esters or other acid derivatives in their structures (e.g., amides, carbamates, ureas, etc., see Table 17.3) are often readily hydrolyzed by microorganisms. To understand how enzymatic steps can be used to transform these substances, it is instructive to consider the hydrolases (i.e., enzymes that catalyze hydrolysis reactions) used by organisms to split naturally occurring analogs (e.g., fatty acid esters in lipids or amides in proteins). The same chemical processes, and possibly even some of the same enzymes themselves, are involved in the hydrolysis of xenobiotic substrates. 

Amides undergo an acid- or base-catalyzed hydrolysis reaction with water in the same way that esters do. Just as an ester yields a carboxylic acid and an alcohol, an amide yields a carboxylic acid and an amine (or ammonia). The net effect is a substitution of -N by -OH. This hydrolysis of amides is the key process that occurs in the stomach during digestion of proteins. 

In addition, in living systems, most biochemical reactions, including ATP hydrolysis, take place during the catalysis of enzymes. The catalytic action of enzymes allows the hydrolysis of proteins, fats, oils, and carbohydrates. As an example, one may consider proteases, enzymes that aid digestion by hydrolyzing peptide bonds in proteins. They catalyze the hydrolysis of interior peptide bonds in peptide chains, as opposed to exopeptidases, another class of enzymes, that catalyze the hydrolysis of terminal peptide bonds, liberating one free amino acid at a time. 

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