Kinetic study on the successive four-step reduction of Cyt c3

Area of Science:

• Biochemistry
• Biophysical Chemistry
• Spectroscopy

Background:

• Cytochrome c3 is a multi-heme protein crucial for electron transfer.
• Understanding the reduction kinetics of its four hemes is essential for elucidating its biological function.

Purpose of the Study:

• To kinetically analyze the successive four-step reduction of cytochrome c3.
• To determine the individual rate constants for each electron transfer step.
• To investigate the reactivity of the four heme groups within the protein.

Main Methods:

• Stopped-flow UV-visible (SF-UV) spectroscopy to monitor absorbance changes.
• Stopped-flow circular dichroism (SF-CD) spectroscopy to track conformational changes.
• Computer simulations to estimate rate constants (k1-k4).

Main Results:

• Rate constants for the four reduction steps (k1=19.8 s-1, k2=11.9 s-1, k3=8.9 s-1, k4=1.6 s-1) were determined.
• Observed rate constants significantly deviate from statistical values, indicating non-equal heme reactivities.
• A small autoacceleration effect was noted for the third reduction step (k3).

Conclusions:

• The reduction of cytochrome c3 hemes is not random; each heme possesses unique reactivity influenced by its local environment.
• The observed autoacceleration suggests specific heme-heme or heme-environment interactions.
• Unusual spectral features support the proposed interactions driving the autoacceleration.