The folding of alpha-1-proteinase inhibitor: kinetic vs equilibrium control

Impact of the PEG length and PEGylation site on the structural, thermodynamic, thermal, and proteolytic stability of mono-PEGylated alpha-1 antitrypsin

Conjugation to polyethylene glycol (PEG) is a widely used approach to improve the therapeutic value of proteins essentially by prolonging their body residence time. PEGylation may however induce changes in the structure and/or the stability of proteins and thus on their function(s). The effects of PEGylation on the thermodynamic stability can either be positive (stabilization), negative (destabilization), or neutral (no effect). Moreover, various factors such as the PEG length and PEGylation site can influence the consequences of PEGylation on the structure and stability of proteins. In this study, the effects of PEGylation on the structure, stability, and polymerization of alpha1-antitrypsin (AAT) were investigated, using PEGs with different lengths, different structures (linear or 2-armed) and different linking chemistries (via amine or thiol) at two distinct positions of the sequence. The results show that whatever the size, position, and structure of PEG chains, PEGylation (a) does not induce significant changes in AAT structure (either at the secondary or tertiary level); (b) does not alter the stability of the native protein upon both chemical- and heat-induced denaturation; and (c) does not prevent AAT to fully refold and recover its activity following chemical denaturation. However, the propensity of AAT to aggregate upon heat treatment was significantly decreased by PEGylation, although PEGylation did not prevent the irreversible inactivation of the enzyme. Moreover, conjugation to PEG, especially 2-armed 40 kDa PEG, greatly improved the proteolytic resistance of AAT. PEGylation of AAT could be a promising strategy to prolong its half-life after infusion in AAT-deficient patients and thereby decrease the frequency of infusions.

Smoothing a rugged protein folding landscape by sequence-based redesign

The rugged folding landscapes of functional proteins puts them at risk of misfolding and aggregation. Serine protease inhibitors, or serpins, are paradigms for this delicate balance between function and misfolding. Serpins exist in a metastable state that undergoes a major conformational change in order to inhibit proteases. However, conformational labiality of the native serpin fold renders them susceptible to misfolding, which underlies misfolding diseases such as α₁-antitrypsin deficiency. To investigate how serpins balance function and folding, we used consensus design to create conserpin, a synthetic serpin that folds reversibly, is functional, thermostable, and polymerization resistant. Characterization of its structure, folding and dynamics suggest that consensus design has remodeled the folding landscape to reconcile competing requirements for stability and function. This approach may offer general benefits for engineering functional proteins that have risky folding landscapes, including the removal of aggregation-prone intermediates, and modifying scaffolds for use as protein therapeutics.

Dynamic local unfolding in the serpin α-1 antitrypsin provides a mechanism for loop insertion and polymerization

The conformational plasticity of serine protease inhibitors (serpins) underlies both their activities as protease inhibitors and their susceptibility to pathogenic misfolding and aggregation. Here, we structurally characterize a sheet-opened state of the serpin α-1 antitrypsin (α₁AT) and show how local unfolding allows functionally essential strand insertion. Mutations in α₁AT that cause polymerization-induced serpinopathies map to the labile region, suggesting that the evolution of serpin function required sampling of high risk conformations on a dynamic energy landscape.

Aeropin from the extremophile Pyrobaculum aerophilum bypasses the serpin misfolding trap

Serpins are metastable proteinase inhibitors. Serpin metastability drives both a large conformational change that is utilized during proteinase inhibition and confers an inherent structural flexibility that renders serpins susceptible to aggregation under certain conditions. These include point mutations (the basis of a number of important human genetic diseases), small changes in pH, and an increase in temperature. Many studies of serpins from mesophilic organisms have highlighted an inverse relationship: mutations that confer a marked increase in serpin stability compromise inhibitory activity. Here we present the first biophysical characterization of a metastable serpin from a hyperthermophilic organism. Aeropin, from the archaeon Pyrobaculum aerophilum, is both highly stable and an efficient proteinase inhibitor. We also demonstrate that because of high kinetic barriers, aeropin does not readily form the partially unfolded precursor to serpin aggregation. We conclude that stability and activity are not mutually exclusive properties in the context of the serpin fold, and propose that the increased stability of aeropin is caused by an unfolding pathway that minimizes the formation of an aggregation-prone intermediate ensemble, thereby enabling aeropin to bypass the misfolding fate observed with other serpins.

Quantitative isolation of alphalAT mutant Z protein polymers from human and mouse livers and the effect of heat

Alpha-1-antitrypsin (alpha1AT) deficiency in its most common form is caused by homozygosity for the alpha1AT mutant Z gene. This gene encodes a mutant Z secretory protein, primarily synthesized in the liver, that assumes an abnormal conformation and accumulates within hepatocytes causing liver cell injury. Studies have shown that mutant alpha1ATZ protein molecules form unique protein polymers. These Z protein polymers have been hypothesized to play a critical role in the pathophysiology of liver injury in this disease, although a lack of quantitative methods to isolate the polymers from whole liver has hampered further analysis. In this study, we demonstrate a quantitative alpha1ATZ polymer isolation technique from whole liver and show that the hepatocellular periodic acid-Schiff-positive globular inclusions that are the histopathological hallmark of this disease are composed almost entirely of the polymerized alpha1ATZ protein. Furthermore, we examine the previously proposed but untested hypothesis that induction of alpha1ATZ polymerization by the heat of physiological fever is part of the mechanism of hepatic alpha1ATZ protein accumulation. The results, however, show that fever-range temperature elevations have no detectable effect on steady-state levels of intrahepatic Z protein polymer in a model in vivo system. In conclusion, methods to separate insoluble protein aggregates from liver can be used for quantitative isolation of alpha1ATZ protein polymers, and the effect of heat from physiological fever may be different in vivo compared with in vitro systems.

A novel mode of polymerization of alpha1-proteinase inhibitor

Patients homozygous for the Z mutant form of alpha1-proteinase inhibitor (alpha1-PI) have an increased risk for the development of liver disease because of the accumulation in hepatocytes of inclusion bodies containing linear polymers of mutant alpha1-PI. The most widely accepted model of polymerization proposes that a linear, head-to-tail polymer forms by sequential insertion of the reactive center loop (RCL) of one alpha1-PI monomer between the central strands of the A beta-sheet of an adjacent monomer. This model derives primarily from two observations: peptides that are homologous with the RCL insert into the A beta-sheet of alpha1-PI monomer and this insertion prevents alpha1-PI polymerization. Normal alpha1-PI monomer does not spontaneously polymerize; however, here we show that the disulfide-linked dimer of normal alpha1-PI spontaneously forms linear polymers in buffer. The monomers within this dimer are joined head-to-head. Thus, the arrangement of monomers in these polymers must be different from that predicted by the loop-A sheet model. Therefore, we propose a new model for alpha1-PI polymer. In addition, polymerization of disulfide-linked dimer is not inhibited by the presence of the peptide even though dimer appears to interact with the peptide. Thus, RCL insertion into A beta-sheets may not occur during polymerization of this dimer.