Multiple molecular recognition mechanisms. Cytochrome P450--a case study.

Biomolecular recognition is complex. The balance between the different molecular properties that contribute to molecular recognition, such as shape, electrostatics, dynamics and entropy, varies from case to case. This, along with the extent of experimental characterization, influences the choice of appropriate computational approaches to study biomolecular interactions. Here, we present computational studies of cytochrome P450 enzymes and their interactions with small molecules and with other proteins. These interactions exemplify some of the diversity of molecular determinants of binding affinity and specificity observed for proteins and we discuss some of the challenges that they pose for molecular modelling and simulation.

RIP death domain structural interactions implicated in TNF-mediated proliferation and survival.

Death domain (DD)-containing proteins are involved in both apoptosis and survival/proliferation signaling induced by activated death receptors. Here, a phylogenetic and structural analysis was performed to highlight differences in DD domains and their key regulatory interaction sites. The phylogenetic analysis shows that receptor DDs are more conserved than DDs in adaptors. Adaptor DDs can be subdivided into those that activate or inhibit apoptosis. Modeling of six homotypic DD interactions involved in the TNF signaling pathway implicates that the DD of RIP (Receptor interacting protein kinase 1) is capable of interacting with the DD of TRADD (TNFR1-associated death domain protein) in two different, exclusive ways: one that subsequently recruits CRADD (apoptosis/inflammation) and another that recruits NFkappaB (survival/proliferation).

Comparative structural and energetic analysis of WW domain-peptide interactions.

WW domains are small globular protein interaction modules found in a wide spectrum of proteins. They recognize their target proteins by binding specifically to short linear peptide motifs that are often proline-rich. To infer the determinants of the ligand binding propensities of WW domains, we analyzed 42 WW domains. We built models of the 3D structures of the WW domains and their peptide complexes by comparative modeling supplemented with experimental data from peptide library screens. The models provide new insights into the orientation and position of the peptide in structures of WW domain-peptide complexes that have not yet been determined experimentally. From a protein interaction property similarity analysis (PIPSA) of the WW domain structures, we show that electrostatic potential is a distinguishing feature of WW domains and we propose a structure-based classification of WW domains that expands the existent ligand-based classification scheme. Application of the comparative molecular field analysis (CoMFA), GRID/GOLPE and comparative binding energy (COMBINE) analysis methods permitted the derivation of quantitative structure-activity relationships (QSARs) that aid in identifying the specificity-determining residues within WW domains and their ligand-recognition motifs. Using these QSARs, a new group-specific sequence feature of WW domains that target arginine-containing peptides was identified. Finally, the QSAR models were applied to the design of a peptide to bind with greater affinity than the known binding peptide sequences of the yRSP5-1 WW domain. The prediction was verified experimentally, providing validation of the QSAR models and demonstrating the possibility of rationally improving peptide affinity for WW domains. The QSAR models may also be applied to the prediction of the specificity of WW domains with uncharacterized ligand-binding properties.

Do mammalian cytochrome P450s show multiple ligand access pathways and ligand channelling?

Understanding substrate binding and product release in cytochrome P450 (CYP) enzymes is important for explaining their key role in drug metabolism, toxicity, xenobiotic degradation and biosynthesis. Here, molecular simulations of substrate and product exit from the buried active site of a mammalian P450, the microsomal CYP2C5, identified a dominant exit channel, termed pathway (pw) 2c. Previous simulations with soluble bacterial P450s showed a different dominant egress channel, pw2a. Combining these, we propose two mechanisms in CYP2C5: (i) a one-way route by which lipophilic substrates access the enzyme from the membrane by pw2a and hydroxylated products egress along pw2c; and (ii) a two-way route for access and egress, along pw2c, for soluble compounds. The proposed differences in substrate access and product egress routes between membrane-bound mammalian P450s and soluble bacterial P450s highlight the adaptability of the P450 fold to the requirements of differing cellular locations and substrate specificity profiles.

Yeast Kre1p is GPI-anchored and involved in both cell wall assembly and architecture.

Kre1p is a cell surface O-glycoprotein involved in a late stage of 1,6-beta-glucan formation in the yeast Saccharomyces cerevisiae. Disruption of KRE1 leads to a 40 % reduction in the overall 1,6-beta-glucan content of the cell wall. This paper shows that in a yeast Deltakre1 null mutant, neither an N-terminal-truncated Kre1p nor Kre1p variants lacking a C-terminal glycosylphospatidylinositol (GPI) attachment site are capable of achieving normal function in glucan assembly, while full-length Kre1p completely complements a Deltakre1 null mutation and restores cell wall 1,6-beta-glucan content up to wild-type level. In a yeast gpi1 mutant, a green-fluorescent-protein-tagged Kre1p derivative is secreted into the medium, indicating an at least transient GPI-anchoring stage of Kre1p during its processing within the yeast secretory pathway. In contrast to the severe defect in cell wall beta-d-glucan, the amount of cell wall mannoproteins is not significantly decreased in a Deltakre1 disruptant, as could be confirmed in competition assays by investigating toxin binding to isolated cell wall mannoproteins. Since the yeast Deltakre1 mutant differed in its sensitivity to zygocin and K28, two killer viral protein toxins that use different cell wall mannoprotein populations as a primary toxin receptor, it can be concluded that in a yeast Deltakre1 background, mannoproteins do not differ significantly in total amount from a Kre1+ wild-type but rather in their expression and distribution at the cell surface. Taken together, these data favour and suggest a structural, rather than enzymic, function of Kre1p in yeast cell wall assembly.

Solution structure of the pore-forming protein of Entamoeba histolytica.

Amoebapore A is a 77-residue protein from the protozoan parasite and human pathogen Entamoeba histolytica. Amoebapores lyse both bacteria and eukaryotic cells by pore formation and play a pivotal role in the destruction of host tissues during amoebiasis, one of the most life-threatening parasitic diseases. Amoebapore A belongs to the superfamily of saposin-like proteins that are characterized by a conserved disulfide bond pattern and a fold consisting of five helices. Membrane-permeabilizing effector molecules of mammalian lymphocytes such as porcine NK-lysin and the human granulysin share these structural attributes. Several mechanisms have been proposed to explain how saposin-like proteins form membrane pores. All mechanisms indicate that the surface charge distribution of these proteins is the basis of their membrane binding capacity and pore formation. Here, we have solved the structure of amoebapore A by NMR spectroscopy. We demonstrate that the specific activation step of amoebapore A depends on a pH-dependent dimerization event and is modulated by a surface-exposed histidine residue. Thus, histidine-mediated dimerization is the molecular switch for pore formation and reveals a novel activation mechanism of pore-forming toxins.

The matrix metalloproteinase 9 (mmp-9) hemopexin domain is a novel gelatin binding domain and acts as an antagonist.

Matrix metalloproteinases (MMPs) are involved in the remodeling processes of the extracellular matrix and the basement membrane. Most MMPs are composed of a regulatory, a catalytic, and a hemopexin subunit. In many tumors the expression of MMP-9 correlates with local tumor growth, invasion, and metastasis. To analyze the role of the hemopexin domain in these processes, the MMP-9 hemopexin domain (MMP-9-PEX) was expressed as a glutathione S-transferase fusion protein in Escherichia coli. After proteolytic cleavage, the isolated PEX domain was purified by size exclusion chromatography. In a zymography assay, MMP-9-PEX was able to inhibit MMP-9 activity. The association and dissociation rates for the interaction of MMP-9-PEX with gelatin were determined by plasmon resonance. From the measured rate constants, the dissociation constant was calculated to be K(d) = 2,4 x 10(-8) m, demonstrating a high affinity between MMP-9-PEX and gelatin. In Boyden chamber experiments the recombinant MMP-9-PEX was able to inhibit the invasion of melanoma cells secreting high amounts of MMP-9 in a dose-dependent manner. These data demonstrate for the first time that the hemopexin domain of MMP-9 has a high affinity binding site for gelatin, and the particular recombinant domain is able to block MMP-9 activity and tumor cell invasion. Because MMP-9 plays an important role in metastasis, this antagonistic effect may be utilized to design MMP inhibition-based cancer therapy.