Chapter 4
Application of Biocatalysts
for the Production of Methanol
from Methane
4.1 Introduction
Methane can be converted to methanol at atmospheric temperature and pressure
by utilizing the isolated enzyme methane monooxygenase (MMO) or whole-cell
methane-oxidizing bacteria as a biocatalyst. The utilization of MMO isolated from
bacterial cells has disadvantages, such as the high cost of isolating MMO and the
instability of MMO outside the bacterial cells. These disadvantages should eventually
be overcome by advances in MMO protein engineering, as discussed at the end of this
chapter. However, at present, methanol production using methane-oxidizing bacteria
is more promising than using isolated MMO.
Methane-oxidizing archaea can also oxidize methane anaerobically. The oxidation occurs via a reverse methanogenesis pathway using several types of electron
acceptors, such as sulfate, nitrate, and metal ions [1–6]. However, the use of anaerobic methane-oxidizing archaea as a biocatalyst to produce methanol from methane
has not yet been reported. Therefore, in this section, only methanol production using
methane-oxidizing bacteria is discussed. Representative research into the production
of methanol using methane-oxidizing bacteria is summarized in Table 4.1 [7–29].
Each method is characterized by the type of biocatalyst (bacterial strain), reactor,
methanol dehydrogenase inhibitor, and electron donor used.
The use of a methanol dehydrogenase inhibitor is essential to achieve the extracellular accumulation of methanol when using methane-oxidizing bacteria. After the
oxidation of methane to methanol by MMO, the subsequent metabolic step is the
dehydrogenation of methanol to formaldehyde by methanol dehydrogenase. Thus,
inhibition of this dehydrogenation is crucial, but results in a shortage of the nicotinamide adenine dinucleotide (NADH) required as cellular energy for the synthesis
of various organic compounds required for the metabolism and replication of the
bacterial cells, as well as the hydroxylation of methane. The carbon assimilation
pathway is also affected by methanol dehydrogenase inhibition, as the assimilation
is initiated from formaldehyde. Therefore, not only methanol oxidation inhibitors,
© Springer Nature Singapore Pte Ltd. 2020
T. Baba and A. Miyaji, Catalysis and the Mechanism of Methane Conversion to Chemicals,
https://doi.org/10.1007/978-981-15-4132-2_4
73
Application of Biocatalysts
for the Production of Methanol
from Methane
4.1 Introduction
Methane can be converted to methanol at atmospheric temperature and pressure
by utilizing the isolated enzyme methane monooxygenase (MMO) or whole-cell
methane-oxidizing bacteria as a biocatalyst. The utilization of MMO isolated from
bacterial cells has disadvantages, such as the high cost of isolating MMO and the
instability of MMO outside the bacterial cells. These disadvantages should eventually
be overcome by advances in MMO protein engineering, as discussed at the end of this
chapter. However, at present, methanol production using methane-oxidizing bacteria
is more promising than using isolated MMO.
Methane-oxidizing archaea can also oxidize methane anaerobically. The oxidation occurs via a reverse methanogenesis pathway using several types of electron
acceptors, such as sulfate, nitrate, and metal ions [1–6]. However, the use of anaerobic methane-oxidizing archaea as a biocatalyst to produce methanol from methane
has not yet been reported. Therefore, in this section, only methanol production using
methane-oxidizing bacteria is discussed. Representative research into the production
of methanol using methane-oxidizing bacteria is summarized in Table 4.1 [7–29].
Each method is characterized by the type of biocatalyst (bacterial strain), reactor,
methanol dehydrogenase inhibitor, and electron donor used.
The use of a methanol dehydrogenase inhibitor is essential to achieve the extracellular accumulation of methanol when using methane-oxidizing bacteria. After the
oxidation of methane to methanol by MMO, the subsequent metabolic step is the
dehydrogenation of methanol to formaldehyde by methanol dehydrogenase. Thus,
inhibition of this dehydrogenation is crucial, but results in a shortage of the nicotinamide adenine dinucleotide (NADH) required as cellular energy for the synthesis
of various organic compounds required for the metabolism and replication of the
bacterial cells, as well as the hydroxylation of methane. The carbon assimilation
pathway is also affected by methanol dehydrogenase inhibition, as the assimilation
is initiated from formaldehyde. Therefore, not only methanol oxidation inhibitors,
© Springer Nature Singapore Pte Ltd. 2020
T. Baba and A. Miyaji, Catalysis and the Mechanism of Methane Conversion to Chemicals,
https://doi.org/10.1007/978-981-15-4132-2_4
73