The current GSTs classification distinguishes tree families of GST enzymes based on their cellular localization: cytosolic, microsomal and mitochondrial. These tree family of enzymes are evolutionary unrelated as demonstrated by phylogenetic analysis. Isoforms are considered to belong to a given family if they have more than 40% similarity. Cytosolic GSTs also known as soluble GSTs are dimeric enzymes found in higher concentration in the cytoplasm of many organisms (Boyland and Chasseaud, 1969; Mannervik, 1988).They can form homo or heterodimers and thus contribute to the expansion of possible isoforms. Alternative splicing play a major in the expansion of GST family in insects. For instance the gene AgGSd1 has four biochemically distinct variants (Ranson et al., 1998). The molecular weight of these enzymes family range from 45 to 55 kDa. Thirteen classes of GSTs have been distinguished in this family based upon their sequence homology, and phylogenetic relationship. They are designated by the greck letters: Alpha, Beta, Delta, Epsilon, Zeta, Theta, Mu, Nu, Pi, Sigma, Tau, Phi, and Omega. Six of these classes are found in insects. The classes Delta and Epsilon are insect-specific and account for most of the insect GSTome. Overall, more than 200 cytosolic GSTs have been validated in insects. The criteria for inclusion in a particular class take also into account the chromosomal location, and immunological coss-reativity. For instance the Anopheles gambiae AgGSTE8 was assigned to Epsilon class because of its adjacent position on the same chromosome with seven other Epsilon GSTs (Ding et al., 2003). In Aedes aegypti the Epsilon class consists of eight sequentially arranged genes mapped on chromosome 2.The physiological function of Omega GSTs is unclear, but one suggestion is that they may play a housekeeping role in protecting against oxidative stress by removing S‐thiol adducts from proteins (Board et al., 2000). Microsomal GSTs are membrane bound trimeric enzymes with a molecular weight of approximately 50 KDa found in vertebrate and in a few instance in invertebrates (Hebert et al 1997). They are known as membrane associated proteins in eicosanoids and glutathione metabolism or MAPEG (Jackobson et al., 1996) for short. This family of enzyme is divided in four subgroups designated by the roman numeral I-IV. Mitochondrial GSTs found in the mitochondria and peroxisomes are mammalians specific GST (Pemble et al., 1996, Morel et al., 2004). They belong to the class kappa. They are similar to cytolic GSTs in their structure and function. Some GST enzymes cannot be accurately assigned to any class based on the current criteria used, and thus are designated by the letter u for unclassified. The nomenclature of GST enzymes was difficult in the field in the early days because of the lack of universally accepted terminology for all species. However a nomenclature of human GSTs has been proposed in by Mannervik et al. (1992) and was subsequently adopted for other species (Hayes and Pulford, 1995). In this scheme of nomenclature a GST enzyme is designated by a lower case letter identifiying the species followed by “GST”, followed by an upper case letter denoting the class to which the enzyme belongs and an arabic numeral denoting the subfamily. In some case a lower case letter is used to depict the allelic variant of the same gene. For instance hGSTM1a-1a refers to a the heterodimeric protein in which both the subunits are from the allelic variant “a” of the human class Mu protein from the subfamily1. Likewise agGSTD1-6 is a GST form Anopheles gambiae formed by heterodimerization of Delta class proteins from the subfamilies 1 and 6.
Ad Blocker Detected
Our website is made possible by displaying online advertisements to our visitors. Please consider supporting us by disabling your ad blocker.