8/18 AAs, Peptides, Proteins

There are 300 different AAs in nature, but only _ are found in mammalian proteins

20

These 20 are the result of 20 different side chains (R)

• When they come together to form a polypeptide, there are infinite possible combinations

• Glycine is the simplest AA; R group is a single H

At neutral pH (~7), AAs predominantly exist as dipolar ions known as _

zwitterion

• A molecule or ion with separate positively and negatively charged groups

At ~pH 7, the amino group is _ and the carboxyl group is _

protonated (-NH₃⁺)

deprotonated (-COO^-)

At ~pH 1-2 (acidic), the amino group is _ and the carboxyl group is _

protonated (-NH₃⁺)

protonated (-COOH)

At ~pH 9-10 (alkaline), the amino group _ and the carboxyl group is _

loses a proton (-NH₂)

deprotonated (-COO^-)

AAs can be _ (non-polar) or _ (polar)

hydrophobic, hydrophilic

• Water solubility is influenced by side chain polarity

Hydrophobic AAs:

• consist entirely of alkyl hydrocarbons (e.g., valine) or bulky aromatic side groups (e.g., phenylalanine)

• are insoluble (or slightly soluble) in water

• aggregate to form an insoluble interior of protein away from the aqueous cytoplasm

• do not gain or lose protons, so cannot form hydrogen or ionic bonds

Hydrophilic AAs:

• have ionized or polar side groups

  • Acidic side chains are proton donors and basic AAs are proton acceptors

  • Both contribute to overall charge of protein

• are water-soluble

• are usually found at the surface of a water-soluble protein