Mobilization of iron stored in the interior cavity of BfrB requires electron transfer from the [2Fe–2S] cluster in Bfd to the core iron in BfrB. A crystal structure of the Pseudomonas aeruginosa BfrB:Bfd complex revealed that BfrB can bind up to 12 Bfd molecules at 12 structurally identical binding sites, placing the [2Fe–2S] cluster of each Bfd immediately above a heme group in BfrB [Yao, H., et al. (2012) J. Am. Chem. Soc., 134, 13470–13481]. We report here a study aimed at characterizing the strength of the P. aeruginosa BfrB:Bfd association using surface plasmon resonance and isothermal titration calorimetry as well as determining the binding energy hot spots at the protein–protein interaction interface. The results show that the 12 Bfd-binding sites on BfrB are equivalent and independent and that the protein–protein association at each of these sites is driven entropically and is characterized by a dissociation constant (Kd) of approximately 3 μM. Determination of the binding energy hot spots was carried out by replacing certain residues that comprise the protein–protein interface with alanine and by evaluating the effect of the mutation on Kd and on the efficiency of core iron mobilization from BfrB. The results identified hot spot residues in both proteins [LB68, EA81, and EA85 in BfrB (superscript for residue number and subscript for chain) and Y2 and L5 in Bfd] that network at the interface to produce a highly complementary hot region for the interaction. The hot spot residues are conserved in the amino acid sequences of Bfr and Bfd proteins from a number of Gram-negative pathogens, indicating that the BfrB:Bfd interaction is of widespread significance in bacterial iron metabolism.

Broadband ISDN networks will require high speed, low delay packet switch fabrics in order to meet the expected quality of service requirements. A Priority Knockout Switch (PKS) architecture employing priority based congestion control mechanisms is presented in this papel: This architecture allows the capability to assign qualities of service to connections through per cell or virtual circuit priority mechanisms. Performance studies which use an analytical model for uniform t r a m and simulation models for uniform and bursty trafic are presented. These studies indicate that the PKS architecture provides excellent cell loss rates for high priority trafic at minor expense to the low priority trafic. CMOS implementations of the switch modules which demonstrate the feasibility of the architecture are also described.

Physical quantitiesOrder parameters have been used to extract the information on internal motions of biomolecules from nuclear magnetic relaxation data (29,40). An order parameter measures the degree of spatial restriction of the motion and is de®ned model-independently as (29):

ChemInform Abstract The prochiral cyclohexanones (I) are converted to the axially chiral oxaziridines (IV) which are obtained as a mixture of four possible stereoisomers. This is irradiated in cyclohexane, yielding the diastereomeric lactams (V) and (VI). Further investigations in this series are described in the original paper.

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