Shown above are three organic compounds and some of their structural isomers. Leucine and isoleucine are two different amino acids and glucose and fructose are two different sugars. Compounds that are mirror images of one another are called optical isomers, or enantiomers. But you cannot superimpose them just as you cannot superimpose your left hand on your right hand. Typically an enzyme can act on only one of a pair of enantiomers.
Proteins are polymers made up of amino acids.
This produces a charged but electically neutral zwitterion. Figure 3.4a from the textbook. Shown are ball and stick, and molecular formula models of the molecules. Figure 3.4b from the textbook. All amino acids have fairly similar pKA's for their carboxyl and amino groups.