Why human cytochrome P450c21 is a progesterone 21-hydroxylase

Area of Science:

• Biochemistry
• Enzymology
• Molecular Biology

Background:

• Human cytochrome P450c21 (CYP21A2) is crucial for cortisol and aldosterone biosynthesis, catalyzing the 21-hydroxylation of progesterone (P4) and 17α-hydroxyprogesterone (17OHP4).
• This reaction involves energetically unfavorable radical intermediates, suggesting specific substrate binding to prevent side reactions at more kinetically favorable sites like C-17 and C-16, which are hydroxylated by CYP17A1.

Purpose of the Study:

• To investigate the hypothesis that CYP21A2's substrate-binding pocket restricts access to the C-21 hydrogen atoms, thereby preventing hydroxylation at other sites.
• To explore whether expanding the CYP21A2 substrate-binding pocket could alter its substrate specificity and introduce new hydroxylation activities.

Main Methods:

• Computational modeling of CYP21A2 based on the crystal structure of CYP2C5.
• Molecular dynamics simulations to analyze steroid nucleus binding orientation and heme-oxygen complex access.
• Site-directed mutagenesis of key residues within the CYP21A2 substrate-binding pocket (L107, L109, V470, I471, V359).
• Analysis of P4 metabolite profiles following enzymatic assays with wild-type and mutant CYP21A2 enzymes.

Main Results:

• Molecular dynamics simulations showed steroid binding perpendicular to the heme ring, limiting heme oxygen access to C-21.
• Mutations V470A/G and I471A/G retained P4 21-hydroxylase activity.
• Mutations V359A and V359G introduced significant 16α-hydroxylase activity, producing 40% and 90% of P4 metabolites, respectively.
• Mutations L107 and L109 resulted in inactive enzymes.

Conclusions:

• Progesterone (P4) binds to CYP21A2 in a distinct orientation compared to CYP17A1.
• Expansion of the CYP21A2 substrate-binding pocket through specific mutations can alter substrate trajectories.
• These alterations enable "metabolic switching," leading to the acquisition of novel hydroxylase activities, such as 16α-hydroxylation.