Figure 4.

Absorbance changes of oxidized cytochrome bd-I induced by (NH₄)₂SO₄. (A) Double difference absorption spectra of the isolated cytochrome bd-I (3.2 μM): each spectrum is a difference between two difference spectra, (NH₄)₂SO₄-treated oxidized minus oxidized and K₂SO₄-treated oxidized minus oxidized at the same concentration of the sulfate. The arrows depict the direction of absorbance changes at increasing [(NH₄)₂SO₄]. (B,C) Absorbance changes measured at 426–408 and 623–595 nm as a function of (NH₄)₂SO₄. Experimental data (filled circles) are shown together with their best fits (solid lines) to the hyperbolic equation (see Materials and Methods), giving the values for maximum absorption changes at 426–408 and 623–595 nm A_max of 8.9 ± 0.6 and 0.8 ± 0.1 mM⁻¹cm⁻¹ and K_dapp of 35.9 ± 7.1 and 24.6 ± 12.4 mM (NH₄)₂SO₄ (7.2 ± 1.4 and 4.9 ± 2.5 mM NH₃), respectively (mean ± standard deviation, n = 3; R² = 0.94547 (Soret region), R² = 0.69231 (visible region)).