Download Amino acid

Spencer L. Seager
Michael R. Slabaugh
www.cengage.com/chemistry/seager
Chapter 19:
Proteins
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. -CH3),
2. neutral and polar (e.g. -CH2-OH),
3. basic (e.g. -CH2CH2CH2CH2-NH3+), or
4. acidic (e.g.-CH2-COO-).
COMMON AMINO ACIDS
COMMON AMINO ACIDS (continued)
COMMON AMINO ACIDS (continued)
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.
“R” Group
Isoelectric Point
Neutral
About pH 6
Acidic
Less than pH 6
Basic
More than pH 6
• 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 H3O+ and OH-.
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 3rd and 8th 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.