460 IRON-CONTAINING PROTEINS AND ENZYMES
Methane is the most diffi cult hydrocarbon substrate to oxidize because it has
a high C – H bond energy (104 kcal/mol), no dipole moment, and no functional-
ity that would assist its binding to a catalytic site. Researchers interested in
the methane monooxygenase enzyme believe that it might be harnessed to
convert natural gas, mostly methane, to more useful methanolic fuels and to
oxidize other alkanes to their more useful alcohol counterparts. Wide varieties
of organic molecules are oxidized by this enzyme; for instance, halogenated
hydrocarbons can be oxidized by MMO in bioremediation efforts.^182 The
MMO enzyme of methanotropic bacteria can be utilized to remove the harmful
greenhouse gas methane from the atmosphere, thus playing an environmental
role here as well.
Two types of methane monooxygenases have been studied: (1) soluble
methane monooxygenase (sMMO) and (2) particulate (membrane - bound)
methane monooxygenase (pMMO). The well - studied sMMO is produced by
methanotrophs under copper - limiting conditions. All methanotrophs produce
pMMO — found in intracytoplasmic membranes — but it is the less well - studied
enzyme.
Soluble methane monooxygenase (sMMO) is a multicomponent enzyme
consisting of the following subunits: (1) a 251 - kDa hydroxylase protein
(MMOH) containing a non - heme dinuclear iron center; (2) a 39 - kDa NADH -
dependent, [2Fe – 2S] - and FAD - containing reductase (MMOR); (3) a 16 - kDa
regulatory protein (MMOB); and (4) a protein of unknown function that binds
to MMOH and inhibits sMMO activity (MMOD). The enzyme activates dioxy-
gen for further oxidation chemistry, incorporating one of its oxygens into the
product — hence the name monooxygenase. The sMMO ’ s MMOH component
catalyzes the reaction with methane (converting it to methanol) at an active
site containing a non - heme di - iron moiety according to the following reactions
adapted from reference 183 :
Fe III()( ) ()−− ++→ +OH 22 Fe III 22 2e−+H Fe II() 2 H O (7.16)
2 Fe II()+→O 22 Fe IV( )( )−μO −Fe IV( ) (7.17)
Three enzymes capable of hydroxylating methane are known: soluble methane
monooxygenase (sMMO), particulate methane monooxygenase (pMMO), and
a related enzyme ammonia monooxygenase (AMO). All methanotrophs
produce pMMO — found in intracytoplasmic membranes — but until now it has
been the less well - studied enzyme. Particulate MMO (pMMO) is composed
of three subunits: (1) pmoB ( α , ∼ 47 kDa), (2) pmoA ( β , ∼ 24 kDA), and (3)
pmoC ( γ , ∼ 22 kDa). Each of these subunits contains membrane - spanning
helices. A 100 - kDa αβγ polypeptide arrangement has been proposed, but a
defi nitive molecular mass for the enzyme is not known at this time. The
number of metal centers in pMMO remains controversial, with reported values
of 2 – 15 copper ions and 0 – 2 iron ions per ∼ 100 kDa of purifi ed pMMO.^184 Five
trinuclear copper centers were proposed by Chan on the basis of the hyperfi ne
splitting in the electron paramagnetic resonance (EPR) spectrum ( g = 2.06),