What could be the meaning of the red circles with letters S in them in the diagram below? I searched in the text but could not find.
It almost certainly refers to the amino acid (residue) serine. The numbers (300, 333, 351) refer to the residue numbers. The COOH at the end signifies the C-terminus of the protein (subunit).
The black P's next to it are phosphate groups; both protein kinase A and C ("PKA" and "PKC") are involved in control of protein activity precisely via phosphorylation of serine residues in proteins (amongst others).