Isothermal calorimetry of a monoclonal antibody using a conventional differential scanning calorimeter.

It has been shown that isothermal calorimetry is able to provide critical information regarding the kinetics of denaturation/aggregation of monoclonal antibodies at temperatures below Tm. Those measurements, however, required sophisticated specialized instrumentation. Here, we demonstrate that similar measurements can be performed using widely available conventional differential scanning calorimeters (DSC) when operated in isothermal scan mode. The denaturation/aggregation kinetics of the anti-HIV monoclonal antibody VRC07-523LS was measured by isothermal DSC at ten degrees below Tm. It is shown that a readily available instrument provides similar kinetic information and can become an important tool for determining the long term stability of biologics.

Long term stability of a HIV-1 neutralizing monoclonal antibody using isothermal calorimetry.

Different factors affect the long term stability of monoclonal antibodies, among them denaturation or partial denaturation that is often followed by aggregation. Isothermal calorimetry is capable of quantifying the kinetics of denaturation/aggregation of an antibody by measuring the heat that is released or absorbed by the process over a period of days or weeks, at temperatures below its denaturation temperature, Tm. The denaturation/aggregation kinetics of the anti-HIV monoclonal antibody VRC07-523LS was measured by isothermal calorimetry at different concentrations in four different formulation buffers. The measurements were performed at ten degrees below Tm, as determined by differential scanning calorimetry. The formation of aggregates was also followed by size exclusion chromatography at 5 °C, 25 °C and 40 °C over a period of 8-36 weeks. It was observed that the rates measured by isothermal calorimetry correlate quantitatively with those measured by size exclusion chromatography. Since isothermal calorimetry experiments are performed over a period of ten days, it can become a valuable tool for a fast prediction of the best formulations.

Temperature stability of proteins: Analysis of irreversible denaturation using isothermal calorimetry.

The structural stability of proteins has been traditionally studied under conditions in which the folding/unfolding reaction is reversible, since thermodynamic parameters can only be determined under these conditions. Achieving reversibility conditions in temperature stability experiments has often required performing the experiments at acidic pH or other nonphysiological solvent conditions. With the rapid development of protein drugs, the fastest growing segment in the pharmaceutical industry, the need to evaluate protein stability under formulation conditions has acquired renewed urgency. Under formulation conditions and the required high protein concentration (∼100 mg/mL), protein denaturation is irreversible and frequently coupled to aggregation and precipitation. In this article, we examine the thermal denaturation of hen egg white lysozyme (HEWL) under irreversible conditions and concentrations up to 100 mg/mL using several techniques, especially isothermal calorimetry which has been used to measure the enthalpy and kinetics of the unfolding and aggregation/precipitation at 12°C below the transition temperature measured by DSC. At those temperatures the rate of irreversible protein denaturation and aggregation of HEWL is measured to be on the order of 1 day-1 . Isothermal calorimetry appears a suitable technique to identify buffer formulation conditions that maximize the long term stability of protein drugs.

Conformational stability and self-association equilibrium in biologics.

Biologics exist in equilibrium between native, partially denatured, and denatured conformational states. The population of any of these states is dictated by their Gibbs energy and can be altered by changes in physical and solution conditions. Some conformations have a tendency to self-associate and aggregate, an undesirable phenomenon in protein therapeutics. Conformational equilibrium and self-association are linked thermodynamic functions. Given that any associative reaction is concentration dependent, conformational stability studies performed at different protein concentrations can provide early clues to future aggregation problems. This analysis can be applied to the selection of protein variants or the identification of better formulation solutions. In this review, we discuss three different aggregation situations and their manifestation in the observed conformational equilibrium of a protein.

Denatured state aggregation parameters derived from concentration dependence of protein stability.

Protein aggregation is a major issue affecting the long-term stability of protein preparations. Proteins exist in equilibrium between the native and denatured or partially denatured conformations. Often denatured or partially denatured conformations are prone to aggregate because they expose to solvent the hydrophobic core of the protein. The aggregation of denatured protein gradually shifts the protein equilibrium toward increasing amounts of denatured and ultimately aggregated protein. Recognizing and quantitating the presence of denatured protein and its aggregation at the earliest possible time will bring enormous benefits to the identification and selection of optimal solvent conditions or the engineering of proteins with the best stability/aggregation profile. In this article, a new approach that allows simultaneous determination of structural stability and the amount of denatured and aggregated protein is presented. This approach is based on the analysis of the concentration dependence of the Gibbs energy (ΔG) of protein stability. It is shown that three important quantities can be evaluated simultaneously: (i) the population of denatured protein, (ii) the population of aggregated protein, and (iii) the fraction of denatured protein that is aggregated.

A broad survey reveals substitution tolerance of residues ligating FeS clusters in [NiFe] hydrogenase.

BACKGROUND: In order to understand the effects of FeS cluster attachment in [NiFe] hydrogenase, we undertook a study to substitute all 12 amino acid positions normally ligating the three FeS clusters in the hydrogenase small subunit. Using the hydrogenase from Alteromonas macleodii "deep ecotype" as a model, we substituted one of four amino acids (Asp, His, Asn, Gln) at each of the 12 ligating positions because these amino acids are alternative coordinating residues in otherwise conserved-cysteine positions found in a broad survey of NiFe hydrogenase sequences. We also hoped to discover an enzyme with elevated hydrogen evolution activity relative to a previously reported "G1" (H230C/P285C) improved enzyme in which the medial FeS cluster Pro and the distal FeS cluster His were each substituted for Cys. RESULTS: Among all the substitutions screened, aspartic acid substitutions were generally well-tolerated, and examination suggests that the observed deficiency in enzyme activity may be largely due to misprocessing of the small subunit of the enzyme. Alignment of hydrogenase sequences from sequence databases revealed many rare substitutions; the five substitutions present in databases that we tested all exhibited measurable hydrogen evolution activity. Select substitutions were purified and tested, supporting the results of the screening assay. Analysis of these results confirms the importance of small subunit processing. Normalizing activity to quantity of mature small subunit, indicative of total enzyme maturation, weakly suggests an improvement over the "G1" enzyme. CONCLUSIONS: We have comprehensively screened 48 amino acid substitutions of the hydrogenase from A. macleodii "deep ecotype", to understand non-canonical ligations of amino acids to FeS clusters and to improve hydrogen evolution activity of this class of hydrogenase. Our studies show that non-canonical ligations can be functional and also suggests a new limiting factor in the production of active enzyme.