Molecular Soil Biology 2024, Vol.15, No.1, 1-7 http://bioscipublisher.com/index.php/msb 3 mitochondria-targeted activity to fulfill its role in biotin synthesis (Patton et al., 1998). AtBIO3 was earlier thought to encode the gene encoding desulfurised biotin synthase in the biotin synthesis pathway and the bio3 mutant has a phenotype similar to that of and nutrient-deficient phenotype. However, in-depth studies have revealed that BIO3 and BIO1 produce chimeric BIO3-BIO1 transcripts, and this study confirms that BIO3-BIO1 has a bifunctional site that catalyzes two sequential reactions in the same metabolic pathway (Muralla et al., 2008). In all studies of mutants of biotin synthesis genes, it was confirmed that biotin is an essential vitamin for plant growth and that plants cannot grow in the absence of biotin synthesis and without exogenous biotin additions. In 2000, a yeast mutant lacking the vht1 gene (dvht1) was used to identify possible plant biotin transport proteins, and after complementation of an Arabidopsis cDNA library was screened for a single clone capable of growth in a medium containing low concentrations of biotin. This screen identified sequences with high similarity to sucrose transporters (e.g, AtSUC1, AtSUC2). Functional analysis of the proteins showed that a member of the Arabidopsis family of sucrose transporter proteins (named SUC5; At1g71890) was radiolabelled also confirming that SUC5 can transport biotin (Ludwig et al., 2000). In biotin biosynthesis-deficient (bio1 and bio2) embryos, SUC5 is an essential carrier for the delivery of biotin. 4 Progress in the Study of Plant Biotinases Many enzymes in plants catalyze carboxylation, decarboxylation, and transcarboxylation reactions with biotin as an essential cofactor (Nikolau et al., 2003). Carboxylases typically use bicarbonate ions as the carboxyl donor and organic molecules as the acceptor; decarboxylases typically use organic molecules as the donor and water as the carboxyl acceptor; and transcarboxylases typically use organic molecules as the carboxyl donor and organic molecules as the carboxyl acceptor (Knowles, 1989). Four carboxylases with biotin as a cofactor have been identified, and these carboxylases catalyse the following two-step reaction, with A representing the carboxylated receptor substrate: HCO3− + ATP + Enzyme-biotin → Enzyme-biotin-CO2− +ADP +Pi Enzyme-biotin-CO2− +A → Enzyme-biotin + A-CO2− Acetyl coenzyme A carboxylase (ACCase) first received attention as an important component of the fatty acid biosynthesis pathway, catalyzing the first rate-limiting step of fatty acid synthesis in the presence of ATP and bicarbonate with biotin as a cofactor (Salie and Thelen, 2016). In prokaryotes, green algae, and most plants, this enzyme is a heterologous complex, and its activity is dependent on four distinct subunits, biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP), and α- and β-carboxyltransferases (CT) (Elborough et al., 1996). Embryo-specific overexpression of biotin carboxyl carrier protein 2 (BCCP2) in mature Arabidopsis thaliana seeds inhibits ACCase activity in plastids, thereby altering oil, protein, and carbohydrate composition (Chen et al., 2009). A novel family of proteins in Arabidopsis thaliana, named biotin/lipoic acid-binding structural domain-containing protein family (BADC), was identified in in vivo immunoprecipitation using subunit-specific antibodies. Immunoprecipitation methods demonstrated that this newly identified protein interacts with acetyl coenzyme A carboxylase. Meanwhile, the yeast two-hybrid technique demonstrated that the three protein isoforms of BADC interact with each of the two protein isoforms of BCCP, and this interaction is not biotin-dependent (Salie et al., 2016). Another carboxylase identified with biotin as a cofactor is 3-methylcrotonamide acetyl coenzyme A (MCCase) (Nikolau et al., 2003). MCCase is a complex consisting of the biotin subunit MCCA and the non-biotin subunit MCCB. It catalyzes the ATP-dependent process from 3-methylcrotonamide acetyl coenzyme A (MC-CoA) to 3-methyl glutamyl acetyl coenzyme A (MG-CoA) (McKean et al., 2000). When Arabidopsis was used as the study material, it was found that the catabolism of leucine in the mitochondria was inhibited in the mutant material of MCCA and MCCB, resulting in a significant increase in leucine accumulation in the plant. In addition, the mutant materials of MCCA and MCCB exhibited abnormal flower and cilia development and a significant decrease in mutant seed germination, phenotypes that were attributed to the blocked leucine metabolism (Ding et al., 2012). In the study of Arabidopsis MCCase, it was also found that MCCase is inhibited by exogenous carbohydrates, especially sucrose, a phenomenon that may predict that one of the major physiological roles of MCCase is the maintenance of carbon homeostasis in plants (Che et al., 2002).
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