What could these letters "S" in red circles mean in a biochemical diagram?
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 (&quot;PKA&quot; and &quot;PKC&quot;) are involved in control of protein activity precisely via phosphorylation of serine residues in proteins (amongst others).

https://en.wikipedia.org/wiki/Protein_kinase_A
https://en.wikipedia.org/wiki/Protein_kinase_C