All of the 20 common amino acids have at least pka values

Amino acids are weak acids that have at least 2 pka values. They have two titratable protons that composed of alpha carboxyl (-COOH) and alpha amino (-NH3+), but some amino acids have a third titratable protons with ionizable groups in the R side chain.  In addition, the structure of amino acids in peptides and proteins are determined by the charges of the titratable group and by the R group. 

hormones in blood stream

image: dtc.ucsf.edu

endocrine glands secrete hormones into the blood stream near them; the hormones then travel in our bloodstream until it reaches its destination, called a target cell, in distance parts of the body. in the target tissue, hormones lock onto the target’s plasma membrane, called the receptor site.