Thermodynamics of the self-association of glucagon.

Abstract

In water, glucagon exists in an equilibrium between a trimer in which more tan half of the peptide groups are in an a-helical configuration and a monomer which has a random coil configuration with few a-helical residues. The thermodynamics of this self-association have been evaluated by studying the temperature-and concentration-dependence of the mean residue ellipticity at 220 nm. The enthalpy and entropy changes of association were negative at all temperatures between 50 and 500 and had large negative temperature dependencies. Usually an association that involves nonpolar groups is considered to be driven by a positive entropy term. Such an explanation is not tenable in the case of glucagon. However, i the effects of nonpolar groups on the coil-to-helix transition of a p tide areincluded into the thermodynamic considerations ofhydrophobic interactions, then the negative parameters observed for glucagon association can be readily understood. The hydrophobic interaction is therefore not necessarily controlled by the entropy change because, if there are significant conformational changes, the reaction may be controlled by the enthalpy change. Consequently, the more important parameter characteristic of all hydrophobic reactions is the heat capacity change.Glucagon is a 29-residue polypeptide hormone that is bound to target tissues and activates adenyl cyclase. Treatment of liver plasma membranes with reagents that modify membrane structure (i.e:, digitonin or phospholipase A) inhibits both glucagon binding and stimulation of adenylate cyclase activity (1). It has been suggested from studies of the binding of glucagon fragments that the hydrophobic region at the carboxyl end of the molecule is important for binding and activation of adenylate cyclase (2). Recent x-ray diffraction studies of glucagon crystals revealed the formation of a trimer with very strong hydrophobic interactions between the glucagon chains which are largely a-helical (3). In order to understand these interactions, which may play an important role in glucagon binding to its membrane receptor (4) A recent conformational analysis based on the sequence of glucagon suggests that glucagon can readily fold into different conformations (5). In very dilute solutions, glucagon is largely unfolded with'few stable intramolecular bonds (6-9). With increasing concentration, in dilute alkali glucagon forms timers that are highly a-helical and in acid it forms fibrils whose folding is mainly of ,8-structure (6). In certain organic solvents (ethylene or propylene glycol/water mixtures), glucagon folds into a a-helical structure but does not associate (10). Glucagon also becomes more a-helical when bound to lipids-i.e., cationic detergents (11), phospholipid micelles (12), or bilayers (13).We have evaluated the thermodynamic parameters of glucagon association between 50 and 500. These should be of interest in understanding not only the interactions between glucagon molecules but also other reactions that depend on hydrophobic forces. Typical examples ...

Thermodynamics of assembly of Escherichia coli aspartate transcarbamoylase.

“…Although the combination of enthalpic, entropic, and heat capacity changes observed for the dimerization of glucagon would traditionally be interpreted as resulting from hydro. gen bond formation, these effects were attributed to conformational changes on assembly (43), because the crystal structure indicates that the subunit interactions are primarily hydrophobic. The self-association of hemoglobin monomers to form a2 or 14 is dominated by hydrophobic interactions, as are the interchain interactions at the noncooperative a1-01 interface of the tetramer.…”

Amyloid structure – one but not the same: the many levels of fibrillar polymorphism

“…Several studies have been aimed at characterizing prefibrillar intermediate species of glucagon and oligomeric structures have been reported in AFM studies [61,62]. However, according to data from field flow fractionation [79], NMR [80], SAXS [63] and dynamic light scattering [44], the benign α‐helical trimer, which is in rapid equilibrium with monomers [44,64,81] and crystallizes readily [14,82], is the only oligomeric structure that forms at detectable levels at pH 2.5. It is possible that a shift to pH 5.5, where the molecules have an average charge of +1, could allow glucagon to form more stable toxic oligomeric species.…”

The Secretin/Pituitary Adenylate Cyclase-Activating Polypeptide/ Vasoactive Intestinal Polypeptide Superfamily in the Central Nervous System

“…In aqueous solution, glucagon was found to adopt predominantly an extended flexible conformation containing a local non-random spatial structure from residues 22 to 25 [62][63][64][65][66]. Upon prolonged standing or when subject to solvent-containing media, glucagon adopts a well-defined conformation as well as self-aggregates to form a trimer in the form of anti-parallel β-chains [67,68].…”