Amino Acids and Proteins Biology 12 E. McIntyre

Where do we find proteins? What do gelatin desserts, hair, antibodies, spider webs, blood clots, egg whites, tofu, and fingernails all have in common? They are all made of protein.

Proteins genetic information in DNA codes specifically for the production of proteins 50% of the dry mass of most cells As with carbohydrates and lipids, their three-dimensional structure is directly related to their function enzymes are proteins

Proteins: Basic Characteristics Proteins are amino acid polymers central carbon atom surrounded by an amino group, a carboxyl group, a hydrogen atom, and a variable group of atoms called a side chain, usually symbolized by the letter ‘R’. Remember that carboxyl and amino groups are functional groups. There are 20 different R groups commonly found in living organisms and, so there are 20 different amino acids. General structure of an amino acid

There are 20 amino acids. Note all are the same except for the side chains

Amino acids: General Properties Amino acids are amphiprotic (which means that they possess both acidic (carboxyl) and basic (amino) functional groups.) When dissolved in water, the carboxyl group donates an H ion to the amino group, causing the carboxyl group to become negatively charged and the amino group to possess an extra hydrogen and a net positive charge.

…Amino acids: General Properties Amino acids may be polar (hydrophilic), nonpolar (hydrophobic), or charged (acidic or basic) Acidic amino acids possess a carboxyl group on their side chains, and basic amino acids contain amino groups on their side chains. These characteristics are determined by an amino acid’s side chain.

Examples of Amino Acids

Types of Amino Acids Nonpolar R = H, CH3 O Polar ll R = –CH2OH, –CH2SH, –CH2C–NH2, (polar groups with –O-, -SH, -N-) Acidic R = –CH2COOH, or -COOH Basic R = –CH2CH2NH2

Learning Check AA1 Identify each as polar or nonpolar A. B.

Solution AA1 Identify each as polar or nonpolar A B Non-polar polar

Essential Amino Acids 10 amino acids not synthesized by the body Arginine (arg) histadine (his) isoleucine (ile) leucine (leu) threonine (thr) lysine (lys), methionine (met) phenylalanine (phe) trytophan (trp) valine (val) Must obtain from the diet All in diary products 1 or more missing in grains and vegetables Want to remember the essential amino acids? Any Help In Learning These Little Molecules Proves Truly Valuable

Amino Acids as Acids and Bases Ionization of the –NH2 (amino) and the –COOH (carboxyl) group An amphiprotic molecule has both a + and – charge An amphiprotic molecule is neutral overall Zwitterion of alanine In water, an amino acid loses a H+ ion from the carboxyl group and donates it to the amino group.

pH and ionization H+ OH– + + Positive ion zwitterion Negative ion + + H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO– Positive ion zwitterion Negative ion Low pH neutral pH High pH

Learning Check AA2 CH3 CH3 + H3N–CH–COOH H2N–CH2–COO– (1) (2) (1) (2) Select from the above structures A. Alanine in base. B. Alanine in acid.

Solution AA2 CH3 CH3 + H3N–CH–COOH H2N–CH2–COO– (1) (2) (1) (2) Select from the above structures (2) Alanine in base. (1) Alanine in acid.

Stop & Think Remember that carboxyl and amino groups are functional groups. Think: What are functional groups? How are polymers of amino acids made?

Solution to Stop & Think Carboxyl groups and amino groups react together to link amino acids.

The Peptide Bond Amide bond formed by the –COOH of an amino acid and the –NH2 of the next amino acid O CH3 + | | + | NH3–CH2–COH + H3N–CH–COO– O CH3 + | | | NH3–CH2–C – N–CH–COO– | peptide bond H

Peptides Amino acids linked by amide (peptide) bonds Gly Lys Phe Arg Ser H2N- -COOH end Peptide bonds end Glycyllysylphenylalanylarginylserine

Learning Check AA3 What are the possible tripeptides formed from one each of leucine, glycine, and alanine?

Solution AA3 Tripeptides possible from one each of leucine, glycine, and alanine Leu-Gly-Ala Leu-Ala-Gly Ala-Leu-Gly Ala-Gly-Leu Gly-Ala-Leu Gly-Leu-Ala

Learning Check AA4 Write the three-letter abbreviations for the following tetrapeptide:

Solution AA4 Ala-Leu-Cys-Met

Types of Proteins Type Examples Structural tendons, cartilage, hair, nails Contractile muscles Transport hemoglobin Storage milk Hormonal insulin, growth hormone Enzyme catalyzes reactions in cells Protection immune response

Protein Structure