U01.01.110 Amino acids

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Learning Objectives

  • Identify the Essential Amino Acids that must be acquired through the diet.
  • Distinguish between Acidic and Basic amino acids based on their side chain charge at physiological pH.
  • Explain the metabolic classification of amino acids as Glucogenic or Ketogenic.
  • Recognize the importance of Arginine and Lysine in DNA-binding proteins.

1. Essential Amino Acids

Amino acids are the building blocks of proteins, and in humans, only the L-isomer form is used. Nine of these are considered “essential” because the human body lacks the enzymatic machinery to synthesize their carbon skeletons de novo. Therefore, these must be obtained from dietary protein sources. These amino acids are further classified by their metabolic products; while most are glucogenic (can be converted to glucose), Leucine and Lysine are the only two that are purely ketogenic.

High-Yield Mnemonic:

“PVT TIM HaLL”

  • Phenylalanine, Valine, Tryptophan
  • Threonine, Isoleucine, Methionine
  • Histidine, Leucine, Lysine

Activity

Metabolic Tip:

Purely Ketogenic: Leucine, Lysine.

Purely Glucogenic: Methionine, Histidine, Valine (“We Met His Valentine, who is so sweet“).

Both: Isoleucine, Phenylalanine, Threonine, Tryptophan.

Activity

2. Acidic Amino Acids

Acidic amino acids possess a carboxylic acid group in their side chains. At physiological pH (approximately 7.4), these side chains lose a proton and carry a negative charge. These residues are often found on the surface of proteins or within active sites where they can participate in ionic bonding or facilitate proton transfer.

  • Aspartic acid (Aspartate): A key intermediate in the urea cycle and a precursor for nucleotide synthesis.
  • Glutamic acid (Glutamate): The most abundant excitatory neurotransmitter in the central nervous system.
Key Property: Both are negatively charged at body pH.

3. Basic Amino Acids

Basic amino acids contain nitrogen-rich side chains that can accept protons. At physiological pH, Arginine and Lysine are positively charged, making them highly reactive with negatively charged molecules. Histidine is unique because its pKa is close to physiological pH, meaning it can exist in both charged and uncharged forms, making it a frequent participant in enzyme catalysis.

  • Arginine: The most basic amino acid; it is a precursor for Nitric Oxide (NO) and is essential during periods of rapid growth.
  • Lysine: An essential amino acid used in the biosynthesis of proteins and carnitine.
  • Histidine: Essential for the production of histamine; it has no net charge at body pH but is categorized as basic.

Clinical Correlation: Histones

Arginine and Lysine are found in high concentrations in histones. Their positive charge allows them to bind to the negatively charged phosphate backbone of DNA.

Activity

High-Yield Mnemonic: “His Lys (lies) are basic.”

(Histidine, Lysine, and Arginine are the basic amino acids).

Activity