Chapter 19: Proteins

AMINO ACIDS The building blocks of proteins Only 20 amino acids are common in proteins Called -amino acids because amino group is attached to the -carbon, next to the carboxylate group

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+) 4. acidic (e.g.-CH2-COO-)

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.

ZWITTERIONS, cont. 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 H3O+ and OH-. “R” Group Isoelectric Point Neutral About pH 6 Acidic Less than pH 6 Basic More than pH 6

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

REACTIONS, cont. Peptide bond formation – amino acids can make polymers by forming amide (peptide) linkages

REACTIONS, cont. Peptides – an amino acid polymer of short chain length Polypeptide – intermediate chain length polymer, less than 50 amino acids Protein – polymer with more than 50 amino acids Amino acid residue – 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 – hormones released by pituitary gland Both have disulfide bridges Vasopressin (antidiuretic hormone) decreases urine formation Oxytocin causes uterine contractions.

IMPORTANT PEPTIDES, cont. Adrenocorticotropic hormone – released by pituitary gland Has no disulfide bridges Regulates the production of steroids by the adrenal gland

PROTEIN SIZE

PROTEIN FUNCTIONS

PROTEIN FUNCTIONS

PROTEIN SHAPES Fibrous proteins – long rod-shaped or stringlike molecules that intertwine to form fibers (collagen, elastin, keratin) Globular proteins – spherical-shaped proteins that form stable suspensions in water, or is water soluble (hemoglobin, enzymes)

PROTEIN COMPOSITION Simple proteins – contain only amino acid residues Conjugated proteins – contain amino acid residues and other organic or inorganic components (prosthetic groups)

PRIMARY PROTEIN STRUCTURE The linear sequence of amino acids in a protein chain Determines secondary and tertiary structures

SECONDARY PROTEIN STRUCTURE Determined by hydrogen bonding between amide groups of amino acid residues in the chain. Two basic types of secondary structure: -helix – keratin, myosin, epidermin, Fibrin -pleated sheets – found extensively only in silk protein Random coil molecular structure

-HELIX

-PLEATED SHEET

TERTIARY PROTEIN STRUCTURE The specific 3-D shape of a protein resulting from interactions between “R” groups of amino acid residues “R” group interactions include: Disulfide bridges – form between cysteine residues Salt bridges – ionic bonds form between acidic and basic residues Hydrogen bonds – form between polar residues Hydrophobic interactions – form between nonpolar residues

TERTIARY STRUCTURE, cont.

TERTIARY STRUCTURE, cont. In an aqueous environment, the interaction of hydrophilic and hydrophobic side chains with water also determines shape.

QUATERNARY PROTEIN STRUCTURE All proteins have primary, secondary, and tertiary structure, but not all proteins have quaternary structure. Quaternary structure is the arrangement of subunits that form a larger protein. Subunits are polypeptides that have primary, secondary, and tertiary structure. Conjugated proteins with quaternary structure contain subunits as well as prosthetic groups, which may be organic or inorganic components. Heme is an example of a prosthetic group:

QUATERNARY PROTEIN STRUCTURE Hemoglobin 4 chains (subunits): two identical alpha chains and two identical beta chains 4 heme groups Hydrophobic forces hold subunits together

PROTEIN HYDROLYSIS Heat and acid or base can completely hydrolyze proteins. This is an important process in protein digestion.

PROTEIN DENATURATION The process by which a protein loses its characteristic quaternary, tertiary, and secondary structure Leads to a loss of biological activity (function)

PROTEIN DENATURATION