Importance of the C-terminal alpha-helical structure for glucagon's biological activity

Glucagon receptor antagonist and GIP agonist combination for diet-induced obese mice

Ablation of glucagon receptor signaling represents a potential treatment option for type 2 diabetes (T2DM). Additionally, activation of glucose-dependent insulinotropic polypeptide (GIP) receptor signaling also holds therapeutic promise for T2DM. Therefore, this study examined both independent and combined metabolic actions of desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon (glucagon receptor antagonist) and d-Ala(2)GIP (GIP receptor agonist) in diet-induced obese mice. Glucagon receptor binding has been linked to alpha-helical structure and desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon displayed enhanced alpha-helical content compared with native glucagon. In clonal pancreatic BRIN-BD11 beta-cells, desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon was devoid of any insulinotropic or cAMP-generating actions, and did not impede d-Ala(2)GIP-mediated (P<0.01 to P<0.001) effects on insulin and cAMP production. Twice-daily injection of desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon or d-Ala(2)GIP alone, and in combination, in high-fat-fed mice failed to affect body weight or energy intake. Circulating blood glucose levels were significantly (P<0.05 to P<0.01) decreased by all treatments regimens, with plasma and pancreatic insulin elevated (P<0.05 to P<0.001) in all mice receiving d-Ala(2)GIP. Interestingly, plasma glucagon concentrations were decreased (P<0.05) by sustained glucagon inhibition (day 28), but increased (P<0.05) by d-Ala(2)GIP therapy, with a combined treatment resulting in glucagon concentration similar to saline controls. All treatments improved (P<0.01) intraperitoneal and oral glucose tolerance, and peripheral insulin sensitivity. d-Ala(2)GIP-treated mice showed increased glucose-induced insulin secretion in response to intraperitoneal and oral glucose. Metabolic rate and ambulatory locomotor activity were increased (P<0.05 to P<0.001) in all desHis(1)Pro(4)Glu(9)(Lys(12)PAL)-glucagon-treated mice. These studies highlight the potential of glucagon receptor inhibition alone, and in combination with GIP receptor activation, for T2DM treatment.

Molecular determinants of glucagon receptor signaling

A 29-amino acid polypeptide hormone, glucagon has been one of the most prolific models in the study of hormone action. The key biologic function of glucagon is to counterbalance the actions of insulin and maintain a normal level of serum glucose. Diabetes mellitus can thus be considered a bihormonal disorder with an excess of glucagon contributing to the hyperglycemic state. The effects of glucagon are mediated by the glucagon receptor, which is itself a prototypical member of a distinct category called family B receptors within the G protein-coupled superfamily of seven-helical transmembrane receptors (GPCRs). At the structural level, the peptide ligands of family B receptors are highly homologous, in particular in the N-terminal region of the molecules. The mechanism by which highly homologous peptide ligands selectively recognize their receptors involves distinct molecular interactions that are gradually being elucidated. This review focuses on structural determinants of the glucagon receptor that are important for its activity with respect to interaction with its ligand and G proteins. Information about the glucagon receptor is presented within the context of what is known about other members of the family B GPCRs.

Enkephalin-cell interactions

Enkephalin analogs for multivalent ligand systems, bivalent enkephalins, multivalent enkephalins on polymers, multivalent ligands on vesicles, simultaneous activation of two different receptor systems, and cell interactions of enkephalin/polypeptide conjugates are described. Multivalent ligand systems can trigger receptor-receptor interactions and are considered to possess interesting possibilities in terms of enhanced potency, reduction of side effects, and a new biological activity.

Positively charged residues at positions 12, 17, and 18 of glucagon ensure maximum biological potency

Glucagon is a peptide hormone that plays a central role in the maintenance of normal circulating glucose levels. Structure-activity studies have previously demonstrated the importance of histidine at position 1 and the absolute requirement for aspartic acid at position 9 for transduction of the hormonal signal. Site-directed mutagenesis of the receptor protein identified Asp64 on the extracellular N-terminal tail to be crucial for the recognition function of the receptor. In addition, antibodies generated against aspartic acid-rich epitopes from the extracellular region competed effectively with glucagon for receptor sites, which suggested that negative charges may line the putative glucagon binding pocket in the receptor. These observations led to the idea that positively charged residues on the hormone may act as counterions to these sites. Based on these initial findings, we synthesized glucagon analogs in which basic residues at positions 12, 17, and 18 were replaced with neutral or acidic residues to examine the effect of altering the positive charge on those sites on binding and adenylyl cyclase activity. The results indicate that unlike N-terminal histidine, Lys12, Arg17, and Arg18 of glucagon have very large effects on receptor binding and transduction of the hormonal signal, although they are not absolutely critical. They contribute strongly to the stabilization of the binding interaction with the glucagon receptor that leads to maximum biological potency.

Macromolecular bioactivity: is it resonant interaction between macromolecules?--Theory and applications

Biological processes in any living organism are based on selective interactions between particular biomolecules. In most cases, these interactions involve and are driven by proteins which are the main conductors of any living process within the organism. The physical nature of these interactions is still not well known. This paper represents a whole new view to biomolecular interactions, in particular protein-protein and protein-DNA interactions, based on the assumption that these interactions are electromagnetic in their nature. This new approach is incorporated in the Resonant Recognition Model (RRM), which was developed over the last 10 years. It has been shown initially that certain periodicities within the distribution of energies of delocalized electrons along a protein molecule are critical for protein biological function, i.e., interaction with its target. If protein conductivity was introduced, then a charge moving through protein backbone can produce electromagnetic irradiation or absorption with spectral characteristics corresponding to energy distribution along the protein. The RRM enables these spectral characteristics, which were found to be in the range of infrared and visible light, to be calculated. These theoretically calculated spectra were proved using experimentally obtained frequency characteristics of some light-induced biological processes. Furthermore, completely new peptides with desired spectral characteristics, and consequently corresponding biological activities, were designed.

Roles of aspartic acid 15 and 21 in glucagon action: receptor anchor and surrogates for aspartic acid 9

The discovery of aspartic acid at position 9 in glucagon to be a critical residue for transduction has spurred renewed efforts to identify other strategic residues in the peptide sequence that dictate either receptor binding or biological activity. It also became apparent from further studies that Asp9 operates in conjunction with His1 in the activation mechanism that follows binding to the glucagon receptor. Indeed, it was later demonstrated that the protonatable histidine imidazole is important for transduction. It is likely that the interaction of a positively charged histidine 1 with a negatively charged aspartic acid 9 might be part of the triggering step at the molecular level. Two other aspartic acid residues in glucagon are capable of assuming a similar role, namely that of contributing to an electrostatic attraction with histidine via a negative carboxylate. These studies were conducted to investigate the role of aspartic acid 15 and 21 in glucagon action. Evidence reported here, gathered from 31 replacement analogs, supports the idea that in the absence of the requisite carboxyl group at position 9, histidine utilizes Asp21 or Asp15 as a compensatory site. Asp15 was also found to be indispensable for binding and may serve to tether the hormone to the receptor protein at the binding site. It is also demonstrated that these new findings promote the design of better glucagon antagonists.

New glucagon analogues with conformational restrictions and altered amphiphilicity: effects on binding, adenylate cyclase and glycogenolytic activities

In an effort to obtain highly potent glucagon antagonists, we have investigated glucagon (1) structure-function relationships utilizing the following design principles: (1) structural changes known to lead to partial agonist activities; (2) conformational restrictions; (3) changes in the conformational probabilities of the primary sequence; and (4) increased amphiphilicity. In this report we present the total synthesis, purification, receptor binding, adenylate cyclase activity, in vivo glycogenolytic activity and CD spectrum of the following four glucagon analogues: [Ahx17,18]glucagon (2), [D-Phe4,Tyr5, 3,5-diiodo-Tyr10,Arg12,Lys17,18,Glu21]glucagon (3), [Asp9,Lys12,Lys17,18,Glu21]glucagon 4, and [Glu15,Lys17,18]glucagon 5. Compound 2 binds exclusively to the high affinity receptor and compound 3 was a highly potent antagonist with respect to adenylate cyclase activity. Analog 4 showed distinct biphasic binding (IC50 5.6 nM and 630 nM), with only the low affinity binding leading to adenylate cyclase activity. Furthermore in analogue 5 receptor binding and adenylate cyclase activity were dissociated by a factor of 5. The results are consistent with a multistep binding mechanism in which glucagon interacts first nonspecifically with the anisotropic interphase of the cell membrane, followed by a conformational transition which occurs in the sequences 10-14 and 15-18 when the membrane bound peptide binds to its receptor.