Vitreoscilla hemoglobin. Intracellular localization and binding to membranes

Area of Science:

• Microbiology
• Biochemistry
• Molecular Biology

Background:

• The bacterium Vitreoscilla produces high levels of homodimeric hemoglobin (VHb) under low oxygen conditions.
• Heterologous expression of VHb can improve growth and product formation in other organisms.
• A proposed function for VHb is to facilitate oxygen transfer to respiratory membranes.

Purpose of the Study:

• To investigate the cellular localization and function of Vitreoscilla hemoglobin (VHb).
• To determine the binding characteristics of VHb to bacterial cell membranes.
• To elucidate the interaction between VHb and the respiratory chain components.

Main Methods:

• Immunogold labeling was used to determine VHb localization in Vitreoscilla and recombinant Escherichia coli.
• VHb-OmpA signal peptide fusions were created to test periplasmic transport.
• Binding affinities (K(d)) of VHb to cell membranes were measured.
• The effect of VHb on ubiquinol-1 oxidase activity was assessed.
• Interactions with Vitreoscilla cytochrome bo were studied using proteoliposomes.

Main Results:

• VHb was localized to the cytoplasm, concentrated near the cell membrane, not in the periplasm.
• Periplasmic localization of VHb did not confer additional growth advantages.
• VHb exhibited high-affinity binding to Vitreoscilla and E. coli cell membranes (K(d) ~5-6 µM), exceeding that of horse hemoglobin/myoglobin.
• VHb significantly stimulated ubiquinol-1 oxidase activity in Vitreoscilla membranes by 68%.
• VHb showed increased binding affinity and site number to Vitreoscilla cytochrome bo in proteoliposomes.

Conclusions:

• VHb is a cytoplasmic protein that binds bacterial respiratory membranes with high affinity.
• VHb enhances bacterial respiration, likely by interacting with components like cytochrome bo.
• The findings support a role for VHb in improving oxygen utilization in bacteria.