Amino Acids, Peptides, and Proteins: A Comprehensive Overview, Slides of Biochemistry

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Unit 4

Presented: Tam-Anh Duong Nguyen, PhD

Amino acids, peptides

and proteins

Contents of the lesson

• Part I. Amino acids
• Part II. Peptides and Proteins
• Part III. Enzymes

Learning objectives of Part I

• Describe the structure and properties of

amino acids

• Classify amino acids

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Structure of an amino acid

• Amino acids are the building blocks of proteins.
• 20 amino acids are used to make every protein on earth.
• Each amino acid has a carboxyl group (-COOH), an amino group (- NH 2 ) and a distinctive side chain (R) bonded to the α carbon atom (except proline ).

Glycine, the only amino acid with a hydrogen atom as a side chain

Proline contains a secondary amine group (– NH- ), called an imine, instead of a primary amine group (-NH 2 )

• The two chiral forms of amino acids are called 'D' and 'L’. D and L forms are mirror images of each other ( enantiomers ).
• Only L-amino acids are found in proteins formed biologically (important!!!).

Optical properties of amino acids

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Acid/base properties of amino acids

• Amino acid has both a basic amine groups and an acidic carboxylic acid group (an ampholyte )  amino acids have both acidic and basic properties.

Isoelectric point (pI)

• The pI of a molecule is the pH at which its charge is exactly equal.
• The pI of an amino acid must be calculated precisely as the average of the pKa values on either side of the molecule with a charge of zero.

A titration curve for glycine. The pH controls the charge on glycine: cationic below pH 2.3; zwitterionic between pH 2.3 and 9.6; and anionic above pH 9.6. The isoelectric pH is 6.0.

Classification of amino acids

• Amino acid can be classified based on:
  • Nutritional requirements
  • Metabolic fate
  • The side chain properties
  • Biosynthetic incorporation into protein

• Essential amino acids or

indispensable amino acids

• Non-essential amino acids or

dispensable amino acids

Classification of amino acids based on nutritional requirement

• Ketogenic amino acids
  • A ketogenic amino acid is an amino acid that can

be converted into ketone bodies through

ketogenesis.

• Glucogenic amino acids
  • A glucogenic amino acid is an amino acid that

can be converted into glucose through

gluconeogenesis

• Both ketogenic and glucogenic amino acids :
  • Amino acid can be used for the production of

either glucose or ketone bodies.

Classification of amino acids based on metabolic fate

Amino acids are grouped according to the

properties of their R groups

The R group of amino acids

• Twenty amino acids are commonly found in

proteins.

• The properties of the amino acids are

determined by the composition of the R groups

on each of them.

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