Cysteine peptidase, mechanism

The mechanism of hydrolysis of cysteine peptidases, in particular cysteine endopeptidases (EC 3.4.22), shows similarities and differences with that of serine peptidases [2] [3a] [55 - 59]. Cysteine peptidases also form a covalent, ac-ylated intermediate, but here the attacking nucleophile is the SH group of a cysteine residue, or, rather, the deprotonated thiolate group. Like in serine hydrolases, the imidazole ring of a histidine residue activates the nucleophile, but there is a major difference, since here proton abstraction does not appear to be concerted with nucleophilic substitution but with formation of the stable thiolate-imidazolium ion pair. Presumably as a result of this specific activation of the nucleophile, a H-bond acceptor group like Glu or Asp as found in serine hydrolases is seldom present to complete a catalytic triad. For this reason, cysteine endopeptidases are considered to possess a catalytic dyad (i.e., Cys-S plus H-His+). The active site also contains an oxyanion hole where the terminal NH2 group of a glutamine residue plays a major role. 

Storer, A.C. and Menard, R. (1994) Catalytic mechanism in papain family of cysteine peptidases. Meth. Enzymol., 244, 486. 

Cystatin refers to a diverse family of protein cysteine protease inhibitors. There are three general types of cystatins Type 1 (stefens), which are primarily found in the cytoplasm but can appear in extracellular fluids Type 2, which are secreted and found in most extracellular fluids and Type 3, which are multidomain protease inhibitors containing carbohydrates and that include the kininogens. Cystatin 3 is used to measure renal function in clinical chemistry. See Barrett, A.J., The cystatins a diverse superfamily of cysteine peptidase inhibitors, Biomed. Biochim. Acta 45,1363-1374,1986 Katunuma, N., Mechanisms and regulation of lysosomal proteolysis, Revis. Biol. Cellular 20, 35-61, 1989 Gauthier, F., Lalmanach, G., Moeau, T. et al., Cystatin mimicry by synthetic peptides, Biol Chem. Hoppe Seyler 373, 465-470, 1992 Bobek, L.A. and Levine,... 

The structures of hepatitis A viral 3C proteinases complexed with tetrapeptidyl-based methyl ketone inhibitors were shown to have an episulfide cation embedded in them. The authors concluded that the mechanism of inactivation of 3G peptidases by methyl ketone inhibitors is different than those operating in serine proteinases or in papain-like cysteine peptidases <2006MI673>. 

The peptidases were separated into catalytic types according to the chemical nature of the group responsible for catalysis. The major catalytic types are, thus, Serine (and the related Threonine), Cysteine, Aspartic, Metallo, and As-Yet-Unclassified. An in-depth presentation of catalytic sites and mechanisms, based on this classification, is the subject of Chapt. 3. 

The previous chapter offered a broad overview of peptidases and esterases in terms of their classification, localization, and some physiological roles. Mention was made of the classification of hydrolases based on a characteristic functionality in their catalytic site, namely serine hydrolases, cysteine hydrolases, aspartic hydrolases, and metallopeptidases. What was left for the present chapter, however, is a detailed presentation of their catalytic site and mechanisms. As such, this chapter serves as a logical link between the preceding overview and the following chapters, whose focus is on metabolic reactions. 

Much of the data related to the mechanism of hexachlorobutadiene toxicity indicate that the intermediates produced by modification of the S-1,1,2,3,4-pentachlorodienyl cysteine derivative are responsible for the observed effects on the proximal tubules of the nephrons. The cysteine derivative is formed from the hexachlorobutadiene conjugate in the liver, intestines, and/or kidney through the action of yglutamyl transferase which removes the glutamate from the glutathione tripeptide followed by the action of a peptidase that removes the glycine from the carboxy terminus. 

Probably the most important protective mechanisms involve the tripeptide GSH (chap. 4, Fig. 59). This compound is found in most cells, and in liver cells, it occurs at a relatively high concentration, about 5 mM or more. There are three pools of GSH cytosolic, mitochondrial, and nuclear. GSH structure is unusual for a peptide in the glutamyl, and cysteine residues are not coupled via a peptide bond, hence the molecule is resistance to peptidase attack. It has a nucleophilic thiol group, and it can detoxify substances in one of three ways ... 

This text is a good source of information on the chemical mechanisms underlying the different modes of peptidase catalysis. Three important enzymes are covered subtilisin, a serine endopepti-dase papain, a cysteine endopeptidase and chymosin, an aspartic endopeptidase. 

Proteins can be modified by a group of peptide hydrolyses (peptidases) commonly called proteases (or proteinoses). Based on their ability to hydrolyze specific proteins, proteases are classified as collagenase, keratinase, elastase, etc. On the basis of the pH range over which they are active, they are classified as either acidic, neutral, or alkaline. However, according to their mechanism of action, the Enzyme Commission classifies proteases into the four distinct classes of serine, cysteine, aspartyl, and metalloproteases. Serine proteases, for example, always contain serine residue at their catalytic center, which is essential for the action of proteolysis. 

Peptidases including keratinases are hydrolases able to hydrolyze peptide bonds in proteins and peptides. They are classified using three different approaches (1) the chemical mechanism of catalysis (based on the catalytic amino acid or metal ion at then-active site, represented by serine, cysteine, threonine, aspartic, asparagine, glutamic and metallocatalytic type), (2) the catalytic reaction (this type of classification depends on the selectivity for the bonds that the peptidases will hydrolyze), and (3) the molecular structure and homology. In this latter approach, amino acid... 

Peptidases or proteases are enzymes that hydrolyse peptide bonds [9]. Proteolytic enzymes can be classified in five classes on basis of their catalytic mechanism aspartic, metallo-, cysteine, threonine and serine peptidases, whereby the latter three follow the same basic mechanism (Scheme 7.3) [10], Another classification of peptidases on the basis of statistically significant similarities in amino acid sequences was presented by Rawlings et al. (MEROPS database) [11], Serine proteases (SP) alone cover approximately one-third of all known proteases, and can accelerate the peptide hydrolysis very efficiently 10 fold) [6,11,12], SPs also hydro-... 

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