This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which of the following can occur many times during a protein's life span? 
Select and explain. 
A. Disulfide bond formation, B. Gamma-carboxylation, C. Glycosylation, D. Phosphorylation
D: Phosphorylation
Answer: D: Phosphorylation 
Explanation: A variety of co and post-translational modifications can occur with proteins, including all those listed. Most of these occur only at one point during the formation of a functional protein, but protein phosphorylation (by addition of phosphate by protein kinases), and then later dephosphorylation, can occur many times during an individual protein molecule's life span. Protein phosphorylation is commonly used by the cell to either upregulate or downregulate a protein's activity. Disulfide bond formation occurs during protein formation to stabilize connections between different pas of a protein, so that the protein will not destabilize after cleavage of peptide bonds. Gamma-carboxylation produces sites that bind calcium ions, and is paicularly common among blood clotting factors. Glycosylation is the addition of oligosaccharide as proteins pass through the ER and Golgi apparatus, and is often used by the cell to help direct proteins to paicular destinations.