that many of the ACC deaminase genes have been transmitted through horizontal
gene transfer (HGT) (Hontzeas et al. 2005; Blaha et al. 2006). Furthermore, when the
evolution of acdR (the bacterial gene responsible for the regulation of acdS) was
studied, it was suggested that acdR may have also evolved through HGT, however,
these two genes may have been evolved separately from one another (PrigentCombaret et al. 2008). More recently, Nascimento et al. (2012) proposed that the
acdS gene is transferred between many strains of Mesorhizobium spp. via symbiotic
island exchange. A more extensive study based on protein structure and phylogeny
analyses performed on a vast number of diverse bacterial and fungal genera,
suggested that current ACC deaminase genes share a common ancestor and likely
evolved from site-specific mutations within the ancestral enzyme gene (Nascimento
et al. 2014). Additional mutations in the parental genes over time may justify the lack
of stringency in terms of substrate specificity among similar proteins. It was also
proposed that the vast majority of acdS and acdR genes evolved through vertical
inheritance although HGT may explain the presence of these genes in organisms that
are not involved in interacting with ACC in the environment (Nascimento et al.
2014).
19.4 Protein Biochemistry
Biochemical studies on ACC deaminase reveal that this is exclusively a cytoplasmically localized enzyme and is not secreted (Jacobson et al. 1994). The enzyme
from various microbial origins share amino acid similarities (Prigent-Combaret et al.
2008), encoded by a single gene namely acdS, and contain 325–345 amino acid
residues with a subunit molecular mass of 33–42 kDa (Tables 19.1 and 19.2). The
ACC deaminase is a multimeric enzyme and has been suggested to be functional in
either of homodimer, homotrimer, or of homotetramer forms (Fedorov et al. 2013).
Based on its three-dimensional structure, ACC deaminase enzyme belongs to the
tryptophan synthase beta superfamily and essentially acts as hydrolase (Karthikeyan
et al. 2004). ACC deaminase requires pyridoxal phosphate (PLP) as a cofactor and
deaminates ACC, an immediate precursor of the plant hormone ethylene, into
ammonia and α-ketobutyrate. The cofactor PLP is tightly bound to the enzyme
and the amount of cofactor that is required for the enzyme to become active is a
1:1 enzyme subunit to PLP ratio, for example, 2 moles PLP is needed to make a mole
of dimeric ACC deaminase active and 3 moles of PLP is required to make a mole of
trimeric ACC deaminase active (Honma 1985).
Despite the fact that this enzyme shares at least 60% of its amino acid residues
between diverse bacterial genera (Nascimento et al. 2014), the secondary structures
(mainly alpha helices) of enzymes isolated from various sources are found to be
somewhat different (Hontzeas et al. 2004). At present, the ACC deaminase enzyme
has been well characterized from at least four diverse phylogenetic origins namely
Pseudomonas spp. (Hontzeas et al. 2004; Jacobson et al. 1994; Karthikeyan et al.
2004), yeast (Minami et al. 1998), Methylobacterium spp. (Fedorov et al. 2013), and
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
369
gene transfer (HGT) (Hontzeas et al. 2005; Blaha et al. 2006). Furthermore, when the
evolution of acdR (the bacterial gene responsible for the regulation of acdS) was
studied, it was suggested that acdR may have also evolved through HGT, however,
these two genes may have been evolved separately from one another (PrigentCombaret et al. 2008). More recently, Nascimento et al. (2012) proposed that the
acdS gene is transferred between many strains of Mesorhizobium spp. via symbiotic
island exchange. A more extensive study based on protein structure and phylogeny
analyses performed on a vast number of diverse bacterial and fungal genera,
suggested that current ACC deaminase genes share a common ancestor and likely
evolved from site-specific mutations within the ancestral enzyme gene (Nascimento
et al. 2014). Additional mutations in the parental genes over time may justify the lack
of stringency in terms of substrate specificity among similar proteins. It was also
proposed that the vast majority of acdS and acdR genes evolved through vertical
inheritance although HGT may explain the presence of these genes in organisms that
are not involved in interacting with ACC in the environment (Nascimento et al.
2014).
19.4 Protein Biochemistry
Biochemical studies on ACC deaminase reveal that this is exclusively a cytoplasmically localized enzyme and is not secreted (Jacobson et al. 1994). The enzyme
from various microbial origins share amino acid similarities (Prigent-Combaret et al.
2008), encoded by a single gene namely acdS, and contain 325–345 amino acid
residues with a subunit molecular mass of 33–42 kDa (Tables 19.1 and 19.2). The
ACC deaminase is a multimeric enzyme and has been suggested to be functional in
either of homodimer, homotrimer, or of homotetramer forms (Fedorov et al. 2013).
Based on its three-dimensional structure, ACC deaminase enzyme belongs to the
tryptophan synthase beta superfamily and essentially acts as hydrolase (Karthikeyan
et al. 2004). ACC deaminase requires pyridoxal phosphate (PLP) as a cofactor and
deaminates ACC, an immediate precursor of the plant hormone ethylene, into
ammonia and α-ketobutyrate. The cofactor PLP is tightly bound to the enzyme
and the amount of cofactor that is required for the enzyme to become active is a
1:1 enzyme subunit to PLP ratio, for example, 2 moles PLP is needed to make a mole
of dimeric ACC deaminase active and 3 moles of PLP is required to make a mole of
trimeric ACC deaminase active (Honma 1985).
Despite the fact that this enzyme shares at least 60% of its amino acid residues
between diverse bacterial genera (Nascimento et al. 2014), the secondary structures
(mainly alpha helices) of enzymes isolated from various sources are found to be
somewhat different (Hontzeas et al. 2004). At present, the ACC deaminase enzyme
has been well characterized from at least four diverse phylogenetic origins namely
Pseudomonas spp. (Hontzeas et al. 2004; Jacobson et al. 1994; Karthikeyan et al.
2004), yeast (Minami et al. 1998), Methylobacterium spp. (Fedorov et al. 2013), and
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
369