Structural insights into a key step of brassinosteroid biosynthesis and its inhibition.

Brassinosteroids (BRs) are essential plant steroid hormones that regulate plant growth and development1. The most potent BR, brassinolide, is produced by addition of many oxygen atoms to campesterol by several cytochrome P450 monooxygenases (CYPs). CYP90B1 (also known as DWF4) catalyses the 22(S)-hydroxylation of campesterol and is the first and rate-limiting enzyme at the branch point of the biosynthetic pathway from sterols to BRs2. Here we show the crystal structure of Arabidopsis thaliana CYP90B1 complexed with cholesterol as a substrate. The substrate-binding conformation explains the stereoselective introduction of a hydroxy group at the 22S position, facilitating hydrogen bonding of brassinolide with the BR receptor3-5. We also determined the crystal structures of CYP90B1 complexed with uniconazole6,7 or brassinazole8, which inhibit BR biosynthesis. The two inhibitors are structurally similar; however, their binding conformations are unexpectedly different. The shape and volume of the active site pocket varies depending on which inhibitor or substrate is bound. These crystal structures of plant CYPs that function as membrane-anchored enzymes and exhibit structural plasticity can inform design of novel inhibitors targeting plant membrane-bound CYPs, including those involved in BR biosynthesis, which could then be used as plant growth regulators and agrochemicals.

The crystal structure of heme acquisition system A from Yersinia pseudotuberculosis (HasAypt): Roles of the axial ligand Tyr75 and two distal arginines in heme binding.

Some Gram-negative pathogens utilize an extracellular heme-binding protein called hemophore to satisfy their needs for iron, a metal element essential for most living things. We report here crystal structures of heme acquisition system A from Yersinia pseudotuberculosis (HasAypt) and its Y75A mutant. The wild-type HasAypt structure revealed that the heme iron is coordinated with Tyr75 and a water molecule. The heme-bound water molecule makes extensive hydrogen bond network that includes Arg40 and Arg144 on the distal heme pocket. Arg40, highly conserved for HasAs from Yersinia species, forms a salt bridge with the propionate side chain of heme, and makes π-π stacking and hydrophobic interactions with porphyrin plane. Interestingly, similar Arg-heme interactions are also found for periplasmic heme transporter, PhuT, suggesting that this is an example of a convergent evolution and one of the important interactions for bacterial heme transportation. Heme titration, heme binding kinetics, and the crystal structures of wild-type and Y75A proteins show that, although Tyr75 is primarily important for heme capturing, other interactions with Arg40, Arg144, and hydrophobic residues also contribute for heme acquisition. We also found that HasAypt can form a dimer in solution. The structure of the domain-swapped Y75A HasAypt dimer shows the presence of two low-spin heme molecules coordinated with His84 and His140, and displacement of the Arg40 loop of dimeric Y75A HasAypt results in deformation of the heme-binding pocket. A similar rearrangement of the distal heme loop might occur in heme transfer from HasAypt to HasRypt.