Hi Jacklyn,

I agree. The explanation isn't very descriptive at all. 😔

I'll try my best to explain.

Protein phosphorylation is when a phosphate group is covalently bonded to an amino acid by an enzyme (protein kinase) to activate or deactivate that particular AA.

The MOST COMMON proteins that can be phosphorylated include serine, threonine, tyrosine, and histidine. How do we know that serine is the better answer?

Well, it's all about Compound 1 (given in passage). They describe it in words, starting from left and moving to the right.

(1) Hydrophobic region: long carbon chain on far left

(2) Residues that could be cross-linked: cysteines, which contain thiol groups

(3) Spacer residues: glycines

(4) Phosphorylated residues (DING DING DING): HEY, IT'S SERINE OMG

(5) Residues with an affinity for cells: I don't even care anymore I figured it out

This is why it's so important that you memorize all 20 amino acid groups so that you can easily recognize them.

It's late at night. You open the fridge. Out jumps an amino acid. WHAT IS IT JACKLYN?!

It was serine all along. 😫