Complex sentence: The protein is composed of ~ 300 amino acids residues and has ~ 30 carbohydrate residues attached including 10 sialic acid residues, which are attached to the protein during post-translational modification in the Golgi apparatus.
Split into simpler sentences: The protein is composed of ~ 300 amino acids residues and has ~ 30 carbohydrate residues attached including 10 sialic acid residues. The carbohydrate residues are attached to the protein during its residence in the Golgi apparatus.