Amino acids Organic Chemistry 2

Amino acids Properties 1

• They are crystalline substances; they have an acidic and basic
character, so they have amphoteric behavior.
• They are the elementary units that form proteins.

highest to lowest melting point of Amino Acids, Amides, Carboxylic acids

Amino acids > Amides > Carboxylic acids

Amino acids behave almost like ionic salts—that’s the key reason.

Amino Acids Properties 2

• Chemically they are carboxylic acids with at least one
amino group per molecule.
• There are 20 amino acids that make up human proteins
and each of them has a different group responsible for
its behavior.

Amino Acids Properties 3

• They are nitrogenous carboxylic
acids.
• They are solid, colorless and
crystallizable compounds.
• They have a high melting point.
• Soluble in water due to their
functional groups.
• They are optically active.

Amino acid Optical Activity

• Optical activity appears as the ability to rotate plane-polarized light passing through an amino acid solution

• It originates from the asymmetric (chiral) carbon

• The α-carbon is usually attached to four different substituents (except in glycine)

• This chirality produces the L- and D-configurations of amino acids

Zwitterion

It is a dipolar ion with positive and negative charges.
• The carboxyl group loses a hydrogen ion to become an
anion.
• The amino group accepts a hydrogen ion and becomes a
positively charged ion.

Classification of amino acids

• Acids and their amides: Glutamic, glutamide, aspartic
and asparagine.
• Aliphatic: glycine, valine, isoleucine and leucine.
• Aromatics: phenylalanine, tyrosine and tryptophan.
• Basics: Lysine, histidine and arginine.
• With OH group: serine and threonine
• With sulfur: cysteine, cystine and methionine

Essential amino acids

• These are those that the body cannot synthesize by itself so
they must be obtained through diet.
• Foods that contain protein with all the essential amino acids are
called “high or good quality.”

• “High quality” foods include meat, eggs, dairy products, and
some vegetables such as spelt, soy, and quinoa.
• You can combine (without having to eat it in the same meal)
legume proteins with cereal proteins to obtain all the essential
amino acids.

Essential amino acids (humans)

- Phenylalanine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Threonine
- Tryptophan
- Valine

Peptide Class by quantity

• Peptide:compounds containing two or more
amino acids linked by a peptide bond.
• Polypeptide:The union of many amino acids with a
molecular weight less than 5000
• Proteins:The union of many amino acids with a
molecular weight between 6000 and 40,000,000.

classes of proteins

Simple Proteins

Conjugated Proteins

Simple Proteins

Simple proteins

• Definition: proteins made only of amino acids

• Composition: contain no additional chemical groups

• Examples: Insulin and keratin

  • albumins, globulins, histones

Conjugated Proteins

• Conjugated proteins contain a non-protein component called a prosthetic group

• The prosthetic group may be a carbohydrate, lipid, metal ion, phosphate, or nucleic acid

• Examples include hemoglobin (heme group) and many viral proteins that incorporate nucleic acids or lipids

Levels of Protein Structure

Primary Structure - linear sequence of amino acids

Secondary Structure -alpha helix and Beta-pleated-sheets

Tertiary Structure - 3D shape of single polypeptide chain

Quaternary Structure -Two or more polypeptide chains

Primary Structure

• Sequences of Amino
acids followed by
disulfite bridges.
• Properties
determined by the
main structure

Secondary Structure

Arrangement of the
polypeptide chain in the
space.
– α- helix: spiral folding
of the chain on itself.
– Folded sheet: zig-
zag folded sheet

Tertiary Quaternary

• 3-dimensional Conformation of the proteins.
•Includes secondary structures
• Example: Bone collagen

Quaternary Structure

• Association of two or more
peptide chains that
form the protein

Properties of Proteins

Denaturation, Solubility, Specificity

Denaturation

Definition: Loss of the native 3-D structure of a protein without breaking its primary peptide bonds.
Causes: Heat, pH changes, salts, detergents, organic solvents.
Effect: Loss of biological activity because shape = function.

Solubility

Definition: Capacity of a protein to dissolve in aqueous solution depending on pH and ionic strength.
Key point: Minimum solubility occurs at the isoelectric point (pI).
Influences: Hydrophobic vs hydrophilic residues, salt concentration, pH.

Specificity

Definition: Ability of a protein (especially enzymes) to recognize and act only on a particular substrate or target.
Basis: Structure–function relationship and precise active-site geometry.
Example: Enzymes showing substrate specificity or antibodies recognizing specific antigens.