What is an organic compound found in most cells?

Most cells have proteins, and the basic unit of a protein - that consists in a polymer - are the amino acids.

Specific amino acids act upon substrates--or target compounds--to transform them into new products.

EXAMPLE: AMINO ACIDS IN ENZYMES

Many enzymes can be found within cells. One enzyme is called trypsin, found in the pancreas.

For instance, the SHD catalytic triad is a known trio of amino acids within the enzyme trypsin (which also happens to be a protein) that consists of serine (Ser, S), histidine (His, H), and aspartate (Asp, D).

WHAT DOES EACH AMINO ACID DO HERE?

Serine is supposed to act as a nucleophile (electron donor) to attack a specific carbonyl carbon on the protein, but in order to do that, histidine has to come in as an activator and "activate" serine by convincing it to lose a proton by lowering its pKa (making it more acidic, and thus more willing to lose a proton).

Then, since histidine has a methylene bridge (#-"CH"_2-#) that can freely rotate, it doesn't always face the right way. Since the enzyme is so big, aspartate, being the orientor, has to orient histidine properly so that histidine can activate serine the most effectively.

That, together, allows effective cleavage of specific peptide bonds. Shown immediately above is the first step to that process in this example.