title: Real-time kinetics of the interaction between the two subunits, Escherichia coli thioredoxin and gene 5 protein of phage T7 DNA polymerase.
abstract: T7 phage DNA polymerase is a tight 1:1 complex of the gene 5 protein (g5p) (80 kDa) of phage T7 and thioredoxin (12 kDa) from the Escherichia coli host. The holoenzyme is essential for the replication of the phage. We estimated the real-time kinetics and thermodynamics of the interaction of g5p with thioredoxin (wild type and mutants) using surface plasmon resonance. Thioredoxin was immobilized on a CM5 sensor chip through a six-carbon spacer (6-amino-n-hexanoic acid) using standard amine coupling. Reduced thioredoxin bound g5p but oxidized thioredoxin did not. The association and dissociation phases of the complex fit a two-exponential model with an apparent equilibrium dissociation constant (KD) of 2.2 nm for thioredoxin with 4.7 x 104.M-1.s-1 and 10.5 x 10-5.s-1 as the corresponding association (ka) and dissociation (kd) rate constants. The strong binding of g5p to thioredoxin is therefore due to fast association and very slow dissociation, a situation similar to antigen-antibody interactions. Thioredoxin mutants P34S, D26A, K57M, D26A/K57M, W31F, W31Y, K36A, K36E, and Y49F had KD values in the range of 1 to 8 nm, whereas mutant W28A had a KD of 12.5 nm. No detectable interaction was observed for mutants P40G, W31H, W31A, and C35A. The effect of temperature on KD and the changes in enthalpy (-DeltaH = 20.2 kcal.m-1) and entropy (TDeltaS =-8.4 kcal.m-1) upon formation of the complex suggested that the interaction is driven by an increase in enthalpy and opposed by a decrease in entropy.
authors.affiliation: Medical Nobel Institute for Biochemistry, Karolinska Institute, Stockholm S-171 77, Sweden.
authors.forename: Netai C
authors.initials: NC
authors.lastname: Singha
authors.forename: Alexios
authors.initials: A
authors.lastname: Vlamis-Gardikas
authors.forename: Arne
authors.initials: A
authors.lastname: Holmgren
mesh_terms: D004259:DNA-Directed DNA Polymerase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D019277:Entropy; D004926:Escherichia coli / Q000378:metabolism*; D006358:Hot Temperature; D007700:Kinetics; D009154:Mutation; D011485:Protein Binding; D013816:Thermodynamics; D013879:Thioredoxins / Q000737:chemistry / Q000378:metabolism*; D013997:Time Factors; D018909:src Homology Domains
chemical_list: D013879:Thioredoxins; C034970:bacteriophage T7 induced DNA polymerase; D004259:DNA-Directed DNA Polymerase