24
2 Selective Production of Methanol …
structure of an enzyme plays a role in the control of enzymatic reactions. Therefore,
to understand the mechanism of the conversion of methane to methanol via MMO,
not only the catalytic site of MMO, but also its protein structure and dynamics, must
be understood. To this end, Extensive efforts have been made to obtain the protein
structure of MMO using X-ray crystallography and NMR analysis.
To analyze the protein structure and function of MMO, it has been isolated from the
bacterial cells of methane-oxidizing bacteria. Two types of MMO have been isolated:
soluble MMO (sMMO) and membrane-bound MMO (particulate MMO, pMMO) [1,
2]. sMMO is located in the cytoplasm of methane-oxidizing bacteria, while pMMO
is embedded in the cellular membrane. sMMO and pMMO have completely different
protein structures, but both catalyze the reaction shown in reaction (2.1).
sMMO has been extensively studied, as it is relatively stable under conditions
other from those of biological surroundings (hereafter referred to as in vitro conditions); studies of sMMO are steadily approaching the elucidation of its mechanism
of methane oxidation. On the other hand, pMMO is very unstable in the presence
of molecular oxygen and at room temperature [5–7]. This instability complicates its
structural and functional analysis, and has led to inconsistent experimental results.
Although the methane oxidation mechanism of pMMO remains less well understood
than that of sMMO, the progress in the investigation of pMMO in the twenty-first
century has been remarkable.
This chapter describes the catalytic cycles for the conversion of methane to
methanol by the two MMOs, as well as structural studies of the two MMOs. These catalytic cycles involve tremendously complex molecular mechanisms. Each enzyme
undergoes changes in its protein scaffold in order to interact with four substrates
(methane, molecular oxygen, electrons, and protons), deliver the substrates to the
catalytic site, and control each stage of the catalytic cycle. Thus, some knowledge
of the protein structure and dynamics of these enzymes is necessary to understand
their reaction mechanisms.
2.2 sMMO
2.2.1 Overall Protein Structure
The protein structure and reaction mechanism of sMMO have been thoroughly
studied using sMMO from two bacterial strains, Methylococcus capsulatus (Bath)
and Methylosinus trichosporium OB3b. Although the protein structure and reaction
behavior of the two sMMOs are almost the same, some differences have been found.
sMMO is a protein complex composed of three proteins [8, 9]: Hydroxylase
(MMOH), reductase (MMOR), and component B (MMOB). All three proteins are
required for efficient methane oxidation. The structures of these three proteins are
shown in Fig. 2.1, and their structural properties are briefly summarized in Table 2.1.
2 Selective Production of Methanol …
structure of an enzyme plays a role in the control of enzymatic reactions. Therefore,
to understand the mechanism of the conversion of methane to methanol via MMO,
not only the catalytic site of MMO, but also its protein structure and dynamics, must
be understood. To this end, Extensive efforts have been made to obtain the protein
structure of MMO using X-ray crystallography and NMR analysis.
To analyze the protein structure and function of MMO, it has been isolated from the
bacterial cells of methane-oxidizing bacteria. Two types of MMO have been isolated:
soluble MMO (sMMO) and membrane-bound MMO (particulate MMO, pMMO) [1,
2]. sMMO is located in the cytoplasm of methane-oxidizing bacteria, while pMMO
is embedded in the cellular membrane. sMMO and pMMO have completely different
protein structures, but both catalyze the reaction shown in reaction (2.1).
sMMO has been extensively studied, as it is relatively stable under conditions
other from those of biological surroundings (hereafter referred to as in vitro conditions); studies of sMMO are steadily approaching the elucidation of its mechanism
of methane oxidation. On the other hand, pMMO is very unstable in the presence
of molecular oxygen and at room temperature [5–7]. This instability complicates its
structural and functional analysis, and has led to inconsistent experimental results.
Although the methane oxidation mechanism of pMMO remains less well understood
than that of sMMO, the progress in the investigation of pMMO in the twenty-first
century has been remarkable.
This chapter describes the catalytic cycles for the conversion of methane to
methanol by the two MMOs, as well as structural studies of the two MMOs. These catalytic cycles involve tremendously complex molecular mechanisms. Each enzyme
undergoes changes in its protein scaffold in order to interact with four substrates
(methane, molecular oxygen, electrons, and protons), deliver the substrates to the
catalytic site, and control each stage of the catalytic cycle. Thus, some knowledge
of the protein structure and dynamics of these enzymes is necessary to understand
their reaction mechanisms.
2.2 sMMO
2.2.1 Overall Protein Structure
The protein structure and reaction mechanism of sMMO have been thoroughly
studied using sMMO from two bacterial strains, Methylococcus capsulatus (Bath)
and Methylosinus trichosporium OB3b. Although the protein structure and reaction
behavior of the two sMMOs are almost the same, some differences have been found.
sMMO is a protein complex composed of three proteins [8, 9]: Hydroxylase
(MMOH), reductase (MMOR), and component B (MMOB). All three proteins are
required for efficient methane oxidation. The structures of these three proteins are
shown in Fig. 2.1, and their structural properties are briefly summarized in Table 2.1.