The peptide hormone insulin is composed of two peptide held together primarily by what mechanism? a$.$ charge$-$charge interactions between acidic and basic amino acids b$.$ an amide bond formed from the R$-$groups of a glutamic acid residue and a lysine residue c$.$ hydrophobic interactions d$.$ disulfide bonds between cysteine residues e$.$ extensive hydrogen bonding due to the relatively high serine and glutamine content