Ch20 Proteins: Amino Acids

Types of Amino Acids Classified By Side Chains

(1) Nonpolar Amino Acid (2) Polar Neutral Amino Acid (3) Polar Acidic Amino Acid (4) Polar Basic Amino Acid

Fischer Projection Rules for Amino Acids

(1) The —COOH group is put at the top of the projection formula, the r group at the bottom. This positions the carbon chain vertically. (2) The —NH2 group is in a horizontal position. Positioning it on the left denotes the L-isomer and positioning it on the right denotes the D-isomer.

Amino Acid Properties

- Are white crystalline solids - Have relatively high decomposition points and melting points. Most decompose before the melt. - Most amino acids are not very soluble in water due to strong intermolecular forces within their crystalline structures. - Their intermolecular forces are positive and negative charge centers of zwitterions.

pI problems with three variables

- If R group is Polar Acidic or if Cys or Tyr: Use the two lower pK values for pI - If R group is Polar Basic: Use the two higher pK values for pI

Cysteine (Cys, C)

- Is the only standard amino acid that has a sulfhydryl group (—SH) - The —SH group allows cystein to dimerize by mild oxidizing agent, forming a cystine molecule by covalent disulfide bond. - Cystine can be reduced to form two cystein molecules. - The redox reaction of cysteine is important in regards to protein structure and shape.

Zwitterion

A molecule that has a positive charge on one atom and a negative charge on another atom, but which has no net charge. The net charge on a zwitterion is zero even though parts of the molecule carry charges.

Complete Dietary Protein

A protein that contains all of the essential amino acids in the same relative amounts in which the body needs them.

Essential Amino Acid

A standard amino acid needed for protein synthesis that must be obtained from dietary sources because the human body cannot synthesize it in adequate amounts from other substances.

Alpha-Amino Acid

An amino acid in which the amino group and the carboxyl group are attached to the alpha-carbon atom. The amino acids found in proteins are always these.

Limiting Amino Acid

An essential amino acid that is missing, or present in inadequate amounts, in an incomplete dietary protein.

Amino Acid (definition)

An organic compound that contains both an amino (—NH2) group and a carboxyl group (—COOH).

Incomplete Dietary Protein

An protein that does not contain adequate amounts, relative to the body's needs, of one or more of the essential amino acids.

Complementary Dietary Protein

Are two or more incomplete dietary proteins that, when combined, provide an adequate amount of all essential amino acids relative to the body's needs.

The Two Polar Acidic Amino Acids

Aspartic Acid (Asp, D), Glutamic Acid (Glu, E).

Internal Amino Acid-Base Reaction in Neutral solution

Carboxyl groups tend to lose protons, producing a negatively charged species (—COO-). Amino groups have a tendency to accept protons, producing a positively charged species (—+NH3). The net result is a zwitterion.

The Nine Nonpolar Amino Acids

Glycine (Gly, G), Alanine (Ala, A), Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I), Proline (Pro, P), Phenylalanine (Phe, F), Methionine (Met, M), and Tryptophan (Trp, W).

The Three Polar Basic Amino Acids

Histidine (His, H), Lysine (Lys, K), Arginine (Arg, R).

Standard Amino Acid

One of the 20 a-amino acids normally found in proteins.

The Six Polar Neutral Amino Acids

Serine (Ser, S), Cysteine (Cys, C), Threonine (Thr, T), Asparagine (Asn, N), Glutamine (Gln, Q), Tyrosine (Try, Y).