1. Introduction
Alcohol dehydrogenases (ADHs) from category 1 have been divided into the following: group I long-chain (approximately 350 amino acid residues) zinc-dependent enzymes; group II short-chain (about 250 residues) zinc-independent enzymes; and group III “iron-activated” enzymes that generally contain approximately 385 amino acid residues [1][2]. ADHs’ functions are crucial to the fungal life cycle, such as growing under aerobic or anaerobic conditions, living inside the host, or establishing symbiotic interactions. With the increase in genome sequencing of fungal isolates, ADH systems in fungi belonging to different taxonomic groups have been revealed. However, progress in understanding the molecular structure and physiological role of the various ADH enzymes has been made in very few organisms. The most well-studied ADHs for their role in growth and energy metabolism are ADHs from category 1, group I (Zn-dependent), and group II (Zn-independent proteins).
2. Role of Zn-Dependent ADHs in Growth and Energy Metabolism in Yeasts
The yeast
Saccharomyces cerevisiae is the organism in which the most progress has been made in relation to the identification and study of the function of ADH proteins so that the proteins encoded by the
ADH1 to
ADH7 genes have been identified and biochemically and physiologically characterized.
S. cerevisiae is a Crabtree-positive organism in which the availability of oxygen is irrelevant to fermentative metabolism. Glucose generates high levels of glycolytic enzymes and represses respiration, which leads to ethanol production
[3]. In this yeast, both in the presence and absence of oxygen, the cytosolic enzyme Adh1p produces ethanol and NAD
+ using glucose as a carbon source. Under aerobic conditions, after glucose depletion, the ethanol produced is oxidized to acetaldehyde via the cytosolic enzyme Adh2p; this last compound can be metabolized via the tricarboxylic acid cycle or as an intermediate metabolite in gluconeogenesis
[4][5]. In
S. cerevisiae, the Adh3p enzyme is localized in the mitochondria and was proposed to be involved in the transport of mitochondrial-reducing equivalents to the cytosol
[6]. By using quadruple mutants in
ADH genes, in which only a single functional ADH was left, it was shown that the enzyme Adh3p produces ethanol from acetaldehyde with a capacity no greater than that shown by Adh2p. In the absence of glucose, Adh3p enables growth and ethanol assimilation, which indicates that the product of the
ADH3 gene replaces the Adh1p and Adh2p enzymes in its function, which occurs to a limited extent and is only noticeable when these enzymes are not active
[5]. The Adh4p protein is not similar in amino acid sequence or structurally with the enzymes Adh1p, Adh2p, and Adh3p; it is activated by zinc and resembles Adh1p in its kinetic parameters
[7]. Subsequent studies, using a quadruple mutant that only has Adh4p as a functional enzyme, showed that this enzyme does not allow the production of ethanol via the reduction of acetaldehyde during growth in glucose medium. A similar experimental approach showed that the enzyme Adh5p does not support growth in ethanol as the sole carbon source, indicating that this enzyme is not appreciably involved in the oxidation of ethanol to acetaldehyde
[5]. The
ADH6 gene from
S. cerevisiae encodes a zinc-containing ADH. The enzyme showed specificity to NADPH and activity with aldehydes and primary, aliphatic (linear and branched chain), and aromatic alcohols. The interruption of the
ADH6 gene makes
S. cerevisiae sensitive to toxic concentrations of veratraldehyde, and the overexpression of the
ADH6 gene enables the yeast to grow under these conditions
[8]. The
ADH7 gene of
S. cerevisiae codes for an NADP(H)-dependent ADH, like the Adh6p enzyme. The purified Adh7p enzyme is a homodimer that exhibits broad substrate specificity similar to Adh6p. The deletion of the
ADH7 gene does not affect growth, although with the mutant deleted in this gene, no experiments were carried out on the phenotype of sensitivity to compounds such as veratraldehyde and anisaldehyde, which are produced during ligninolysis and substrates for the enzymes Adh6p and Adh7p
[8].
In the galactose-fermenting yeast
Kluyveromyces lactis, four
ADH genes have been identified that encode cytosolic (
KlAdh1p and
KlAdh2p) and mitochondrial isozymes (
KlAdh3p and
KlAdh4p)
[9][10][11]. The physiological role of
K. lactis ADH enzymes was investigated by generating triple-mutant strains in
ADH genes, in which only a single functional ADH was left
[12]. The analysis of these mutants showed that each ADH activity enables growth in ethanol as the sole carbon source; only the strain carrying all four mutant genes showed an inability to grow in ethanol. Regarding the accumulation of ethanol produced in a culture medium containing glucose, different amounts were observed,
KlAdh1p being the highest producer, followed by
KlAdh2p, and finally,
KlAdh3p and
KlAdh4p produced ethanol at a very low level
[12].
The yeast
Pichia stipitis (currently known as
Scheffersomyces stipitis) is one of the few organisms capable of fermenting the pentose xylose, although it is also capable of using glucose; the yeast shows the regulation of the transition between the respiratory and fermentative processes
[13].
S. stipitis is a Crabtree-negative organism, so oxygen limitation, rather than the presence of glucose or xylose, induces fermentation
[14][15]. Using gene-disruption experiments, the physiological role of the
ADH-gene-encoded products was studied. The loss of
SsADH1 negatively affects growth and ethanol production in a xylose medium under oxygen limitation
[16]. On the other hand, the loss of
SsADH2 enzyme function did not cause the aforementioned effects. The double-mutant strain
SsADH1
SsADH2 could not grow in xylose and produced very little ethanol. These observations suggested that the
SsADH2 isoenzyme is also involved in xylose fermentation
[16]. Like
S. stipitis,
Candida maltosa is capable of fermenting glucose or xylose. In this yeast, three ADH genes,
CmADH1, CmADH2A, and
CmADH2B, were cloned and sequenced
[17]. In a mutant of
C. maltose isolated as resistant to allyl alcohol, it was observed that the activity of the
CmADH2A enzyme was affected and that this deficiency caused a dramatic decrease in ethanol production in a medium with xylose under oxygen limitation; however, the above-mentioned deficiency of
CmADH2A enzyme activity had no effect on ethanol production from glucose under aerobic conditions
[17]. According to the expression pattern of
CmADH isozymes and fermentation studies of the
CmADH2A-deficient mutant, it was found that
CmADH1 and
CmADH2A were both expressed and involved in ethanol formation during xylose metabolism
[17].
Candida utilis has also been known as a yeast capable of metabolizing xylose; the enzyme ADH1 has been described in this organism. Inspection of the amino acid sequence of
C. utilis ADH1 and enzyme characterization with different aliphatic and branched alcohols indicated that it could be a primary alcohol dehydrogenase that requires zinc ions for catalytic reactions; the specific ADH activity is nine times higher for NAD
+ than that for NADP
+, indicating that the enzyme preferred NAD
+ to NADP
+ as a cofactor
[18].
The yeast
Yarrowia lipolytica can utilize ethanol as a carbon source but is not capable of producing ethanol or growing under anaerobic conditions. Eight
ADH genes were identified in this yeast, all of which were deleted
[19][20]. The characterization of combinations of mutations in the
ADH genes showed that the joint alteration of the
ADH1,
ADH2, and
ADH3 genes results in the phenotype of an inability to grow in ethanol, indicating that the corresponding proteins participate in the oxidation of the alcohol and that it is enough that one of them is active for this process to occur. Additionally, it was observed that for growth in ethanol, in addition to the three
ADH genes mentioned, the
ACS1 gene is required, which encodes an acetyl-CoA synthetase
[21]. On the other hand, another study found that alcohol dehydrogenase genes
ADH1 and
ADH3 and a fatty alcohol oxidase gene,
FAO1, play an important role in the oxidation of exogenous fatty alcohols but play less prominent roles in the oxidation of fatty alcohols derived from
n-alkanes
[20].
3. Role of Zn-Dependent ADHs in Growth and Energy Metabolism in Filamentous Fungi
Neurospora crassa was one of the first filamentous fungi in which the ability to produce appreciable amounts of ethanol was demonstrated in cultures incubated in a glucose medium
[22]. Through zymography, it was shown that the fungus produces two different ADHs. Fermentative ADH, which showed a single band of protein with activity, is produced when the fungus is cultured in sucrose, while oxidative ADH, which consists of at least two electromorphs, is produced in a medium with ethanol as a carbon source; the latter enzyme is repressed by glucose or by sucrose. The enzymes also differ in substrate specificity, in the ratio of the forward (glucose as substrate) and reverse reactions (ethanol as substrate), and in their thermostability
[23]. It has been shown that
N. crassa can produce ethanol in cultures with different hexoses and pentoses, as well as with cellulose polymers and with lignocellulosic agroindustrial residues, such as straw, wood, and various agricultural and wood processing waste products
[22]. Genes encoding
ADH1 and
ADH3 enzymes have been identified and expressed in
E. coli to carry out the biochemical characterization of the enzymes
[24]. However, no alteration experiments have been carried out on these genes to demonstrate in vivo the function of the enzymes they encode.
In
Aspergillus nidulans, three genes that code for ADH enzymes have been identified. The
alcA gene codes for the enzyme ADHI, which is the enzyme required to utilize ethanol as the sole carbon source
[25][26][27]. The enzyme alcohol dehydrogenase II (ADHII) has been detected, by its activity, in zymograms, although its physiological function is unknown. This enzyme is encoded by the
alcB gene
[28]. The gene
alcC encoding the enzyme alcohol dehydrogenase III (ADHIII) was identified as a cDNA sequence capable of complementing the
ADHI mutation in
S. cerevisiae [29]. To date, there is no known physiological function of the enzyme.
In
Aspergillus flavus Furukawa et al.,
[30] reported that ethanol and 2-propanol at low concentrations (<1% and <0.6%, respectively) increase aflatoxin production. Both alcohols are incorporated into the aflatoxin biosynthesis via acetyl-CoA. In this context,
A. flavus utilizes ethanol and 2-propanol as carbon sources for aflatoxin biosynthesis, and the
adh1 gene and probably other putative alcohol dehydrogenase controls this aflatoxin production, balancing ethanol production and catabolism.
Mucor circinelloides f. lusitanicus (currently known as
Mucor lusitanicus) is a dimorphic fungus belonging to the subphylum Mucoromycotina. It has been described that
M. lusitanicus is a Crabtree-positive organism since, when grown in a high-glucose medium under aerobic conditions, it produces appreciable amounts of ethanol
[31][32]. In this organism, the
adh1 gene was identified, which encodes a zinc-dependent enzyme that uses NAD
+ as a cofactor
[33]. A mutant of
M. lusitanicus altered in the
adh1 gene showed, in comparison with the wild-type strain R7B and with the mutant complemented with the wild-type
adh+ allele, alteration to several physiological characteristics, including an inability to grow under anaerobic conditions in a glucose medium, production of low levels of ethanol in a medium with glucose in cultures carried out under aerobic conditions, a notable reduction in growth in a medium with ethanol as the only carbon source under aerobic conditions, and very low levels of alcohol dehydrogenase activity in the cytosolic fraction of the mycelium obtained under aerobic conditions in a glucose medium. These observations indicate that in
M. lusitanicus, the product of the
adh1 gene acts to mediate the Crabtree effect and can act as a fermentative or oxidative enzyme, depending on the nutritional conditions
[34].
Shah et al.
[32] suggests that the
adh1 gene from
M. circinelloides is also involved in lipid biosynthesis. In the
M. lusitanicus strain WJ11, the knocking out of the
adh1 gene reduced the ethanol production by 85–90%, and the lipid and fatty acid content was decreased. These lipid and fatty acid contents were restored when the fermentation media was supplemented with 0.5% external ethanol.
In the phytopathogenic fungus
Fusarium oxysporum f. sp.
lycopersici, mutants deficient in the
ADH1 gene were isolated by obtaining spontaneous allyl alcohol-resistant mutants in a glucose medium. The characterization of one of these strains showed that the insertion into the
ADH1 gene of an incomplete transposable element of
F. oxysporum occurred. This mutation caused an inability to use ethanol as a carbon source under aerobic conditions, impaired growth under hypoxic conditions with glucose as a carbon source, and decreased ethanol production in the glucose medium. Complementation with the wild-type
ADH1 allele restored all defects produced by the mutation, indicating that depending on the culture conditions, the
ADH1 gene product has fermentative and oxidative enzymatic functions
[35].
Botrytis cinerea is a necrotrophic fungus that infects many types of plant species. A
Bcadh1 gene was identified in this organism, encoding a zinc-dependent enzyme. A mutant strain deleted in the referred gene was obtained and characterized
[36]. It was observed that, compared to the wild-type strain, the Δ
bcadh1 mutant showed several phenotypic alterations, including a reduced growth capacity in low oxygen tension, greater tolerance to ethanol, and sensitivity to reactive oxygen species. The increased ethanol tolerance of the Δ
bcadh1 mutant indicated that the enzyme is involved in the fermentation of glucose to ethanol. However, the phenotypic characteristic that best explains this physiological role of the enzyme is the lower growth capacity in low oxygen tension
[36].
Table 1 summarizes the substrates, cofactors, oligomeric state, cellular location, and function of fungal Zn-dependent ADH enzymes.
Table 1. Features of Zn-dependent fungal ADHs.
4. Role of Non-Zn-Dependent ADHs in Growth and Energy Metabolism
The white rot basidiomycete fungus
Phanerochaete chrysosporium can degrade lignin to carbon dioxide, which requires enzymes that carry out the reduction of aryl-aldehydes to the corresponding alcohols. In this organism, the
PcAAD1 gene encoding a 386-amino-acid aryl alcohol dehydrogenase (AAD) was identified. The
PcAAD1 gene is expressed in a higher proportion under conditions of nitrogen limitation
[43][45]. Analysis of the protein
PcAad1p indicates that it belongs to the AKR9A subfamily of the AKR (aldo-keto reductase) superfamily, in which there is a conserved NADPH-binding domain. The purified enzyme is active in the reduction of more than 20 different aliphatic and aryl aldehydes, although it shows the highest activity with veratryl alcohol, vanillyl alcohol, and the corresponding aldehydes
[43]. A similar AAD enzyme is overexpressed in the white rot basidiomycete
Coriolus versicolor exposed to either 4-methyldibenzothiophene-5-oxide (4MDBTO) or dibenzothiophene-5-oxide (DBTO)
[46]. It is proposed that in these organisms, aryl alcohol dehydrogenase is part of the lignin degradation machinery and the response to chemical stress
[43][46].
Anaerobic chytridiomycete fungi, such as
Piromyces and
Neocallimastix, are important symbionts in the gastrointestinal tracts of herbivorous mammals, where they contribute to the degradation of plant polymers that form an important part of the diets of ruminants
[47]. It was shown that
Piromyces and
Neocallimastix have an
ADHE-type gene in their genome and that the ADHE protein was located in the cytosol
[47]. This enzyme is homologous to the ADHE protein of eubacteria and that from parabasal flagellate anaerobes
Trichomonas vaginalis and
Tritrichomonas fetus [48].
Piromyces sp. E2 conducts the final steps of its carbohydrate catabolism via PFL and ADHE enzymes, producing metabolites of a bacterial-type mixed-acid fermentation. Similar to bacterial ADHE, the
Piromyces ADHE enzyme is anticipated to be iron-dependent for catalysis and uses NAD as a cofactor
[49].
This entry is adapted from the peer-reviewed paper 10.3390/cells12182239