What is the relationship between amino acids and proteins? ?

where in your body is there an enzyme that does function at a high or low ph? what is the name of that enzyme and its function?

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.