Serpin crystal structure and serpin polymer structure.

Serpins are a family of structurally homologous proteins having metastable native structures. As a result, a serpin variant destabilized by mutation(s) has a tendency to undergo conformational changes leading to inactive forms, e.g., the latent form and polymer. Serpin polymers are involved in a number of conformational diseases. Although several models for polymer structure have been proposed, the actual structure remains unknown. Here, we provide a comprehensive list of serpins, both free and in complexes, deposited in the Protein Data Bank. Our discussion focuses on structures that potentially can contribute to a better understanding of polymer structure.

Thermal stabilization of human albumin by medium- and short-chain n-alkyl fatty acid anions.

A comprehensive study of the thermal stabilization of defatted human albumin monomer by n-alkyl fatty acid anions (FAAs), formate through n-decanoate, was carried out by differential scanning calorimetry (DSC). The concentration of each ligand affording maximum thermal stabilization was determined; n-nonanoate provides the greatest stabilization but is only marginally better than n-octanoate and n-decanoate. The use of reversible thermodynamics and a two-state denaturation model for albumin has been validated. Standard free energies of binding, calculated from increases in free energy of denaturation, for n-butanoate and longer FAAs, are linear with n-alkyl chain length whereas those for formate, acetate, and n-propionate deviate from linearity; those for acetate and n-propionate are even greater than that of n-butanoate, thereby suggesting, in addition to the common class of sites available to all such ligands, the presence of an additional class of lower affinity binding sites available only to these shortest ligands. Competition experiments involving acetate and n-octanoate and involving n-pentanoate and n-octanoate confirmed the binding of acetate to lower affinity sites unavailable to n-octanoate and n-pentanoate. Furthermore, an equation is provided, allowing computation of the transition temperature as a function of the free energy for any reversible process causing a change in thermal stability of a protein undergoing reversible, two-state denaturation. With this equation, modeling the competition experiments by using the binding parameters determined by DSC provides additional support for the class of lower affinity sites, which play a significant role in thermal stabilization of albumin at higher concentrations of these shortest FAAs.

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.