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Poli 3-hydroxyalcanoates (PHA) are the main components of the cytoplasmatic inclusions, called PHA granules. They serve as carbon and energy reserves in many species of procariotes. PHA polymers can be made from different sized monomers, ranging from 4 to 14 carbon atoms. They are typically synthetized when there is an unbalanced distribution of nutrients in the medium, mainly an abundance of carbon sources coupled with the lack of another essential component. Among the existing PHAs, the most common are the poli-3-hydroxybutirate (PHB) polymers. (Barbosa, Gomez, Torres, 2018, p. 52). PHBs are specially relevant due to their properties that resemble conventional plastics, such as polypropylene. Since they are biopolymers made from renewable, biodegrabable and compatible source materials, PHBs present themselves as an industrial alternative to petrol-based plastics. (Byrom, 1987; Madison and Huisman 1999).

The polymerization of PHA monomers is performed by the PHA sintase/polymerase (phaC) enzyme. At first, two acetyl-CoA molecules are condensed into one acetoacetyl-CoA by a β-ketoacyl-CoA thiolase, and then reduced to (R)-3-hydroxybutyryl-CoA by the acetoacetyl-CoA dehydrogenase. Finally, phaC polymerizes the (R)-3-hydroxybutyryl-CoA molecules into the PHA polymer. (Reddy et al., 2003).

Currently, there are four known classes of phaC, that are distinguished by their primary structure, substrate specificity and subunits composition (Rhem, 2008; Neoh et al., 2022). Despite vast diversity, only the catalytic domain of the PhaC_cn-CAT from Ralstonia eutropha H16 and the USM2 PhaC_cs-CAT from the Chromobacterium sp., both being class 1 phaCs. (Neoh et al., 2022).

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Class 1

Overview

The class 1 phaCs synthetize preferentially short chain PHAs, with 3 to 5 carbon monomers, and are composed by a single enzymatic unit, with molecular mass ranging from 63 to 73 kDA. (Chek et al., 2018; Neoh et al., 2022).

Thanks to the X-ray cristalography data from PhaC_cn-CAT and PhaC_cs-CAT, it was possible to categorize the class 1 phaCs in based on their molecular organization, made of two domains: The N-terminal domain and the catalytic C-terminal domain. The C-terminal domain carries the catalytic site, formed by the aminoacid triad (Cys-Asp-His) located deep within the hydrophobic cavity, that in the closed conformation is partially covered by the Cap subdomain.(Chek et al., 2018; Neoh et al., 2022).

N-terminal domain

The N-terminal domain has no known function, and attempts to perform X-ray cristalography of this region have not been sucessful. Many studies have gathered evidence of possible roles that the N-terminal domain performs, such as: enzymatic activity efficiency, binding to PHA granules, substrate specificity, phaC expression, interaction with other PHA-related proteins and dimers formation and estabilization. Still, elucidation of its exact catalytic mechanism remains necessary.(Neoh et al., 2022).

Disease

Relevance

Structural highlights

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