Adrenal microsomal hydroxylating system: purification and substrate binding properties of cytochrome P-450C-21

Area of Science:

• Biochemistry
• Enzymology
• Protein interactions

Background:

• Cytochrome P-450C-21 is crucial for steroidogenesis.
• Understanding its substrate binding is key to its function.
• Previous studies suggested complex binding kinetics.

Purpose of the Study:

• To investigate the substrate-cytochrome P-450C-21 binding reaction.
• To elucidate the role of enzyme concentration and self-association in binding.
• To determine the kinetic parameters of binding and dimerization.

Main Methods:

• Detailed kinetic analysis of substrate-cytochrome P-450C-21 binding.
• Use of purified cytochrome P-450C-21.
• Pattern search procedure for calculating dissociation constants.

Main Results:

• Apparent substrate dissociation constant (KDapp) varied with enzyme concentration.
• Binding data fit a model of monomer-dimer equilibrium where substrate binds only to monomers.
• Intrinsic dissociation constant (K1) and dimerization constant (K2) were independent of enzyme concentration.
• Factors promoting dimer dissociation decreased KDapp, approaching K1.

Conclusions:

• Cytochrome P-450C-21 exists in a monomer-dimer equilibrium.
• Substrate binding is specific to the monomeric form.
• There is a mutual interaction between substrate binding and self-association of cytochrome P-450C-21.