Glucose oxidase (GOX; β-d-glucose:oxygen oxidoreductase) from Aspergillus niger is a dimeric flavoprotein with a molecular mass of 80 kDa/monomer. Thermal denaturation of glucose oxidase has been studied by absorbance, circular dichroism spectroscopy, viscosimetry, and differential scanning calorimetry. Thermal transition of this homodimeric enzyme is irreversible and, surprisingly, independent of GOX concentration (0.2–5.1 mg/ml). It has an apparent transition temperature of 55.8 ± 1.2 °C and an activation energy of ∼280 kJ/mol, calculated from the Lumry-Eyring model. The thermally denatured state of GOX after recooling has the following characteristics. (i) It retains ∼70% of the native secondary structure ellipticity; (ii) it has a relatively low intrinsic viscosity, 7.5 ml/g; (iii) it binds ANS; (iv) it has a low Stern-Volmer constant of tryptophan quenching; and (v) it forms defined oligomeric (dimers, trimers, tetramers) structures. It is significantly different from chemically denatured (6.67 m GdmHCl) GOX. Both the thermal and the chemical denaturation of GOX cause dissociation of the flavin cofactor; however, only the chemical denaturation is accompanied by dissociation of the homodimeric GOX into monomers. The transition temperature is independent of the protein concentration, and the properties of the thermally denatured protein indicate that thermally denatured GOX is a compact structure, a form of molten globule-like apoenzyme. GOX is thus an exceptional example of a relatively unstable mesophilic dimeric enzyme with residual structure in its thermally denatured state.

Significance: Proteins are amazing nanomachines that achieve their functionality through a complex three-dimensional folding process. Some proteins can fold within microseconds. Only a few experimental techniques are available for studying folding on this timescale. In this paper, we expanded the use of single-molecule force spectroscopy toward ultrafast folding proteins. Using this technique, we demonstrate that rapid folding transitions of a single villin headpiece (HP35) can be experimentally monitored and analyzed using distribution moment analysis and autocorrelation functions. We show differences in the folding dynamics and cooperativity between wild-type HP35 and a stable mutant. Brownian dynamics simulations reproduce all key features of our experiments.

Enzymes are molecular machines that bind substrates specifically, provide an adequate chemical environment for catalysis and exchange products rapidly, to ensure fast turnover rates. Direct information about the energetics that drive conformational changes is difficult to obtain. We used subnanometre single-molecule force spectroscopy to study the energetic drive of substrate-dependent lid closing in the enzyme adenylate kinase. Here we show that in the presence of the bisubstrate inhibitor diadenosine pentaphosphate (AP5A), closing and opening of both lids is cooperative and tightly coupled to inhibitor binding. Surprisingly, binding of the substrates ADP and ATP exhibits a much smaller energetic drive towards the fully closed state. Instead, we observe a new dominant energetic minimum with both lids half closed. Our results, combining experiment and molecular dynamics simulations, give detailed mechanical insights into how an enzyme can cope with the seemingly contradictory requirements of rapid substrate exchange and tight closing, to ensure efficient catalysis.

Thecis/transisomerization of peptide bonds before proline (prolyl bonds) is a rate-limiting step in many protein folding reactions, and it is used to switch between alternate functional states of folded proteins. Several prolyl isomerases of the FK506-binding protein family, such as trigger factor, SlyD, and FkpA, contain chaperone domains and are assumed to assist protein folding in vivo. The prolyl isomerase activity of FK506-binding proteins strongly depends on the nature of residue Xaa of the Xaa-Pro bond. We confirmed this in assays with a library of tetrapeptides in which position Xaa was occupied by all 20 aa. A high sequence specificity seems inconsistent with a generic function of prolyl isomerases in protein folding. Accordingly, we constructed a library of protein variants with all 20 aa at position Xaa before a rate-limitingcisproline and used it to investigate the performance of trigger factor and SlyD as catalysts of proline-limited folding. The efficiencies of both prolyl isomerases were higher than in the tetrapeptide assays, and, intriguingly, this high activity was almost independent of the nature of the residue before the proline. Apparently, the almost indiscriminate binding of the chaperone domain to the refolding protein chain overrides the inherently high sequence specificity of the prolyl isomerase site. The catalytic performance of these folding enzymes is thus determined by generic substrate recognition at the chaperone domain and efficient transfer to the active site in the prolyl isomerase domain.