Chapter 19: Proteins Spencer L. Seager Michael R. Slabaugh Jennifer P. Harris

AMINO ACIDS Amino acids are the building blocks of proteins. Only 20 amino acids are common in proteins. They are called  -amino acids because amino group is attached to the  -carbon, which is next to the carboxylate group. Each amino acid has a name, three-letter abbreviation, and one-letter abbreviation.

AMINO ACID “R” GROUPS Amino acids are categorized into four groups based on the “R” group characteristics. The “R” group can be: 1. neutral and nonpolar (e.g. -CH 3 ), 2. neutral and polar (e.g. -CH 2 -OH), 3. basic (e.g. -CH 2 CH 2 CH 2 CH 2 -NH 3 + ), or 4. acidic (e.g.-CH 2 -COO - ).

COMMON AMINO ACIDS

COMMON AMINO ACIDS (continued)

AMINO ACID STEREOCHEMISTRY In 19 of the 20 amino acids, the  -carbon is chiral. With few exceptions, the amino acids in living systems are in the L form. Glycine is the achiral amino acid.

AMINO ACID ZWITTERIONS Amino acids exist as zwitterions, a dipolar ion that results from an internal acid-base reaction. Note that the net charge of the zwitterion is zero.

AMINO ACID ZWITTERIONS (continued) The isoelectric point is the pH at which an amino acid has a net charge of zero. At pH values above the isoelectric point, the amino acid has a net negative value. At pH values below the isoelectric point, the amino acid has a net positive value. Amino acid solutions can act as buffers because they react with both H 3 O + and OH -. “R” GroupIsoelectric Point NeutralAbout pH 6 AcidicLess than pH 6 BasicMore than pH 6

AMINO ACID PROPERTIES Properties that result from existing as zwitterions: White crystalline solids Relatively high melting points High water solubility

AMINO ACID REACTIONS Oxidation of cysteine, the only –SH containing amino acid, to form a disulfide (-S-S-) bridge:

AMINO ACID REACTIONS (continued) Peptide bond formation enables amino acids to make polymers by forming amide (peptide) linkages.

AMINO ACID REACTIONS (continued) Dipeptides are compounds made of 2 amino acids joined by peptide linkage. The order of linkage is important because two different dipeptides can be formed when two unique amino acids react.

AMINO ACID REACTIONS (continued) Peptides are an amino acid polymer of short chain length. A polypeptide is an intermediate chain length polymer with less than 50 amino acids. A protein is a polymer with more than 50 amino acids. An amino acid residue is an amino acid that is part of a polymer. By convention, peptides are written with the N-terminal residue on the left and the C-terminal residue on the right.

IMPORTANT PEPTIDES Vasopressin and oxytocin are hormones released by the pituitary gland and classified as nonapetides. Both have disulfide bridges. They differ only in 3 rd and 8 th amino acids. Vasopressin (antidiuretic hormone) decreases urine formation. Oxytocin causes uterine contractions.

IMPORTANT PEPTIDES (continued) Adrenocorticotropic (ACTH) hormone is released by the pituitary gland. ACTH contains 39 amino acids. ACTH has no disulfide bridges. ACTH regulates the production of steroids by the adrenal gland.

IMPORTANT PEPTIDES (continued)

PROTEIN SIZE

PROTEIN SIZE (continued) Proteins: are extremely large natural polymers. have molecular weights of ~6000 – several million u. are too large to pass through cell membranes. are contained inside the normal cells where they were formed. can leak out if cell is damaged by disease or trauma. Protein in urine can indicate damaged kidneys. Heart enzymes in blood can indicate a heart attack. A typical human cell contains 9000 different proteins. The human body contains ~100,000 different proteins.

PROTEIN ACID-BASE PROPERTIES Proteins: take the form of zwitterions. have a characteristic isoelectric point. behave as buffers in solutions. can be in solution or form stable colloidal dispersions. The form depends on the repulsive forces acting between molecules with like charges on their surfaces. Repulsion is weakest at the isoelectric point, when: the net charge is zero; proteins clump and precipitate from solution.