These will be hydrolyzed into free AA before entering blood circulation. Whole protein can only be absorbed by newborn mammals through pinocytosis. This allows the absorption of intact antibodies in the colostrum. Amino acids in the small intestinal mucosal cells are converted into alanine and glutamic acid before they are transported to liver through portal vein. They are converted back to other AA for protein synthesis.
Protein synthesis takes place in every single living cells.
All 20 amino acids are needed for protein synthesis. Protein degradation involves many different proteases, such as calpains and cathepsins. The exact mechanisms are not known. The former will release ammonia which must be disposed of through urea cycle. Transamination is a process which nonessential AA are made.