Residues in Methylosinus trichosporium OB3b methane monooxygenase component B involved in molecular interactions with reduced- and oxidized-hydroxylase component: A role for the N-terminus

Methane monooxygenase (MMO) is a non-heme-iron-containing enzyme which consists of 3 protein components: A hydroxylase (MMOH), an NAD(P)H-linked reductase (MMOR), and a 138-residue regulatory protein, component B (MMOB). Here, NMR spectroscopy has been used to derive interactions between MMOB and reduced and oxidized states of MMOH (245 kDa). Differential broadening of MMOB resonances in ¹H-¹⁵N HSQC spectra acquired at different molar ratios of MMOH indicates interaction of both proteins, with MMOB binding more tightly to oxidized MMOH as observed previously. The most broadened backbone NH resonances suggest which residues in MMOB are part of the MMOH-binding interface, particularly when those residues are spatially close or clustered in the structure of MMOB. Although a number of different residues in MMOB appear to be involved in interacting with oxidized and reduced-MMOH, some are identical. The two most common segments, proximal in the structure of MMOB, are β-strand 1 with turn 1 (residues 36-46) and α-helix 3 going into loop 2 (residues 101-112). In addition, the N-terminus of MMOB is observed to be involved in binding to MMOH in either redox state. This is most strongly evidenced by use of a synthetic N-terminal peptide from MMOB (residues 1-29) in differential broadening ¹H TOCSY studies with MMOH. Binding specificity is demonstrated by displacement of the peptide from MMOH by parent MMOB, indicating that the peptide binds in or near the normal site of N-terminal binding. The N-terminus is also observed to be functionally important. Steady-state kinetic studies show that neither a Δ2-29 MMOB deletion mutant (which in fact does bind to MMOH), the N-terminal peptide, nor a combination of the two elicit the effector functions of MMOB. Furthermore, transient kinetic studies indicate that none of the intermediates of the MMOH catalytic cycle are observed if either the Δ2-29 MMOB mutant or the N-terminal peptide is used in place of MMOB, suggesting that deletion of the N-terminus prevents reaction of reduced MMOH with O₂ that initiates catalysis.