Biochemistry: Amino Acids

Amphoteric Substances

• Amphoteric describes substances that can act as both acids and bases.

Amino Acid Charge and pH

• The amino group carries a positive charge (+) and the COOH group has no charge at a pH below the pKa of the COOH group.

Isoelectric Point

• The isoelectric point (pI) is the pH at which an amino acid has a net electric charge of zero.

Amino Acid Reactions

• Reaction with ammonia (NH3): The COOH group of an amino acid reacts with ammonia to form an amide and water.
• Reaction with a base: The COOH group of an amino acid loses a proton (H+) to form a carboxylate ion (COO-) and water.

Ion Form of Amino Acids

• In solution, amino acids exist as zwitterions, which have both a positive and a negative charge.

Isoelectric Point and pKa

• The amino acid with an isoelectric point equal to the arithmetic mean of the two pKa values is glycine.

Decarboxylation

• Decarboxylation is the removal of a carboxyl group (COOH) from an amino acid, resulting in the formation of an amine and carbon dioxide (CO2).

Peptide bonds

• Peptide bonds link amino acid residues together in proteins through a condensation reaction between the carboxyl group of one amino acid and the amino group of another.

Functions of Amino Acids

• In addition to protein synthesis, amino acids and their derivatives are involved in various functions, including:
  • Neurotransmitters: Some amino acids act as neurotransmitters in the nervous system, such as glutamate and GABA.
  • Hormones: Certain amino acids are precursors to hormones, like tyrosine, a precursor to dopamine, norepinephrine, and epinephrine.

Short Polymers of Amino Acids

• Short polymers of amino acids are called peptides.

Proteins in the Human Body

• Types of proteins found in the human body include:
  • Enzymes: Catalyze biochemical reactions.
  • Antibodies: Part of the immune system, recognizing and neutralizing foreign substances.
  • Hormones: Act as chemical messengers to regulate various bodily functions.
  • Structural proteins: Provide support and structure to cells and tissues.

Hydrolysis of Proteins

• Complete hydrolysis of proteins yields amino acids.

Amino Acid Side Chains

• The side chain attached to an alpha carbon atom in amino acids (excluding proline) is variable and defines the unique properties of each amino acid.

Amino Group at Physiological pH

• At physiological pH, the amino group in amino acids is protonated, carrying a positive charge (+).

Chiral Carbon

• A carbon atom attached to four different groups exhibits chirality, meaning it has a non-superimposable mirror image.

Amino Acid Without Optical Isomers

• The amino acid glycine does not have optical isomers because its alpha carbon is attached to two identical hydrogen atoms.

Proteins and Amino Acids

• Proteins are composed of L-amino acids, which have a specific configuration at the alpha carbon.

Hydrophobic Amino Acids

• Alanine, valine, leucine, and isoleucine share the characteristic of having hydrophobic R groups.

Proline's Uniqueness

• Proline is unique among amino acids because its R group is bonded to the amino group, forming a cyclic structure.

Sulfur-Containing Amino Acid

• Methionine is a sulfur-containing amino acid.

Hydrophobic Interactions

• Hydrophobic interactions help cluster the side chains of alanine, valine, leucine, and isoleucine within proteins, driving their association and exclusion of water molecules.

Glycine's R Group

• Glycine has an R group that is simply a hydrogen atom.

Water Solubility of Amino Acids

• The R groups of certain amino acids, like lysine, are more soluble in water than nonpolar amino acids due to their polar, charged nature.

Ninhydrin Reaction

• When α-amino acids react with ninhydrin, they form a purple-colored product, used for identifying and quantifying amino acids.

Transamination

• In transamination, an amino group is transferred from an amino acid to a keto acid, forming a new amino acid and a new keto acid.

Oxidative Deamination

• Oxidative deamination removes an amino group from an amino acid, producing ammonia (NH3) and a keto acid.

Collagen's Amino Acids

• Glycine, proline, and hydroxyproline are abundant in collagen, the most abundant protein in mammals.

Histone Modifications

• Lysine, found in histones, can undergo modifications such as methylation, phosphorylation, or acetylation, affecting gene expression.

Blood Clotting Protein

• Prothrombin, a plasma protein involved in blood clotting, contains γ-carboxyglutamic acid.

Majority Amino Acids

• L-amino acids are the majority isolated from animals and plants.

Antibiotics and Amino Acids

• D-amino acids are specifically found in certain antibiotics like actinomycin-D.

Ketogenic Amino Acids

• Leucine and lysine are exclusively ketogenic amino acids, meaning they can be converted to ketone bodies.

Glucogenic and Ketogenic Amino Acids

• Isoleucine, phenylalanine, tryptophan, and tyrosine are both ketogenic and glucogenic, meaning they can be converted to glucose and ketone bodies.

Selenocysteine

• Selenocysteine is unique among amino acids because it contains a selenol (-SeH) group instead of a thiol group.

Pyrrolysine

• Pyrrolysine is chemically characterized by its pyrrolysine side chain, which is different from the standard 20 amino acids.

Selenocysteine Incorporation

• Selenocysteine is incorporated during protein translation at the UGA codon, which typically signals termination.

Common Feature of Amino Acids

• Most amino acids are chiral due to the presence of a chiral carbon atom.

Tasteless Amino Acid

• Glycine is typically tasteless.

Common Property of All Amino Acids

• All amino acids except glycine have a chiral alpha carbon.

This quiz explores the role of amino acids in protein synthesis, their participation in cellular functions, and their derivatives.