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Thermodynamics of folding and binding in an affibody:affibody complex
KTH, School of Biotechnology (BIO), Molecular Biotechnology.
KTH, School of Biotechnology (BIO), Molecular Biotechnology.ORCID iD: 0000-0001-9238-7246
2006 (English)In: Journal of Molecular Biology, ISSN 0022-2836, E-ISSN 1089-8638, Vol. 359, no 5, 1305-1315 p.Article in journal (Refereed) Published
Abstract [en]

Affibody binding proteins are selected from phage-displayed libraries of variants of the 58 residue Z domain. Z(Taq) is an affibody originally selected as a binder to Taq DNA polymerase. The anti-Z(Taq) affibody was selected as a binder to Z(Taq) and the Z(Taq):anti-Z(Taq) complex is formed with a dissociation constant K-d = 0.1 mu M. We have determined the structure of the Z(Taq):anti-Z(Taq) complex as well as the free state structures of Z(Taq) and anti-Z(Taq) using NMR. Here we complement the structural data with thermodynamic studies of Z(Taq) and anti-Z(Taq) folding and complex formation. Both affibody proteins show cooperative two-state thermal denaturation at melting temperatures T-M similar to 56 degrees C. Z(Taq):anti-Z(Taq) complex formation at 25 degrees C in 50 mM NaCl and 20 mM phosphate buffer (pH 6.4) is enthalpy driven with Delta H degrees(bind) = -9.0(+/- 0.1) kcal mol(-1). The heat capacity change Delta C-P degrees,(bind) = -0.43(+/- 0.01) kcal mol(-1) K-1 is in accordance with the predominantly non-polar character of the binding surface, as judged from calculations based on changes in accessible surface areas. A further dissection of the small binding entropy at 25 degrees C (-T Delta S degrees(bind) = -0.6(+/- 0.1) kcal mol(-1)) suggests that a favourable desolvation of non-polar surface is almost completely balanced by unfavourable conformational entropy changes and loss of rotational and translational entropy. Such effects can therefore be limiting for strong binding also when interacting protein components are stable and homogeneously folded. The combined structure and thermodynamics data suggest that protein properties are not likely to be a serious limitation for the development of engineered binding proteins based on the Z domain.

Place, publisher, year, edition, pages
2006. Vol. 359, no 5, 1305-1315 p.
Keyword [en]
protein engineering, protein stability, molecular recognition, binding thermodynamics, calorimetry
National Category
Other Industrial Biotechnology
Identifiers
URN: urn:nbn:se:kth:diva-6409DOI: 10.1016/j.jmb.2006.04.041ISI: 000238988400012Scopus ID: 2-s2.0-33746927966OAI: oai:DiVA.org:kth-6409DiVA: diva2:11110
Note
QC 20110118Available from: 2006-11-22 Created: 2006-11-22 Last updated: 2012-03-20Bibliographically approved
In thesis
1. Structural and thermodynamical basis for molecular recognition between engineered binding proteins
Open this publication in new window or tab >>Structural and thermodynamical basis for molecular recognition between engineered binding proteins
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The structural determination of interacting proteins, both as individual proteins and in their complex, complemented by thermodynamical studies are vital in order to gain in-depth insights of the phenomena leading to the highly selective protein-protein interactions characteristic of numerous life processes. This thesis describes an investigation of the structural and thermodynamical basis for molecular recognition in two different protein-protein complexes, formed between so-called affibody proteins and their respective targets. Affibody proteins are a class of engineered binding proteins, which can be functionally selected for binding to a given target protein from large collections (libraries) constructed via combinatorial engineering of 13 surface-located positions of the 58-residue three-helix bundle Z domain derived from Staphylococcal protein (SPA).

In a first study, an affibody:target protein pair consisting of the ZSPA-1 affibody and the parental Z domain, with a dissociation constant (Kd) of approximately 1 µM, was investigated. ZSPA-1 was in its free state shown to display molten globule-like characteristics. The enthalpy change on binding between Z and ZSPA-1 as measured by isothermal titration calorimetry, was found to be a non-linear function of temperature. This nonlinearity was found to be due to the temperature dependent folded-unfolded equilibrium of ZSPA-1 upon binding to the Z domain and, the energetics of the unfolding equilibrium of the molten globule state of ZSPA-1 could be separated from the binding thermodynamics. Further dissection of the binding entropy revealed that a significant reduction in conformational entropy resulting from the stabilization of the molten globule state of ZSPA-1 upon complex formation could be a major reason for the moderate binding affinity.

A second studied affibody:target complex (Kd ~ 0.1 µM) consisted of the ZTaq affibody protein originally selected for binding to Taq DNA polymerase and the anti-ZTaq affibody protein, selected for selective binding to the ZTaq affibody protein, thus constituting an "anti-idiotypic" affinity protein pair. The structure of the ZTaq:anti-ZTaq affibody complex as well as the free state structures of ZTaq and anti-ZTaq were determined using NMR spectroscopy. Both ZTaq and anti-ZTaq are well defined three helix bundles in their free state and do not display the same molten globule-like behaviour of ZSPA-1. The interaction surface was found to involve all of the varied positions in helices 1 and 2 of the anti-ZTaq, the majority of the corresponding side chains in ZTaq, and also several non-mutated residues. The total buried surface area was determined to about 1670 Å2 which is well inside the range of what is typical for many protein-protein complexes, including antibody:antigen complexes. Structural rearrangements, primarily at the side chain level, were observed to take place upon binding. There are similarities between the ZTaq:anti-ZTaq and the Z:ZSPA-1 structure, for instance, the binding interface area in both complexes has a large fraction of non-polar content, the buried surface area is of similar size, and certain residues have the same positioning. However, the relative orientation between the subunits in ZTaq:anti-ZTaq is markedly different from that observed in Z:ZSPA-1. The thermodynamics of ZTaq:anti-ZTaq association were investigated by isothermal titration calorimetry. A dissection of the entropic contributions showed that a large and favourable desolvation entropy of non-polar surface is associated with the binding reaction which is in good agreement with hydrophobic nature of the binding interface, but as in the case for the Z:ZSPA-1 complex a significant loss in conformational entropy opposes complex formation.

A comparison with complexes involving affibody proteins or SPA domains suggests that affibody proteins inherit intrinsic binding properties from the original SPA surface. The structural and biophysical data suggest that although extensive mutations are carried out in the Z domain to obtain affibody proteins, this does not necessarily affect the structural integrity or lead to a significant destabilization.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 57 p.
Keyword
protein structure, induced fit, binding thermodynamics, NMR spectroscopy, protein engineering, protein-protein interactions, protein stability, calorimetry
National Category
Other Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-4181 (URN)91-7178-481-0 (ISBN)
Public defence
2006-12-01, FR4, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00
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Note
QC 20110118Available from: 2006-11-22 Created: 2006-11-22 Last updated: 2011-12-08Bibliographically approved

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