Amino acids & R Groups

What is the general structure of an amino acid, and what are its main components?

An amino acid has a central carbon atom called the alpha carbon (Cα), bonded to four different groups:

1. Amino group – written as –NH₂ or –NH₃⁺ (at physiological pH, it carries a positive charge).

2. Carboxyl group – written as –COOH or –COO⁻ (at physiological pH, it carries a negative charge).

3. Hydrogen atom – a single hydrogen attached to the alpha carbon.

4. R group (side chain) – the variable group that determines the amino acid’s identity and properties.

What is the role of the R group in amino acids, and how does it affect protein function?

The R group (side chain) determines:

• Chemical properties (e.g. polarity, charge, hydrophobicity).

• Interactions between amino acids, such as hydrogen bonds, ionic bonds, or hydrophobic interactions.

• Folding and 3D structure of the final protein.

• Functionality, as the active sites of enzymes and binding sites are determined by the nature of R groups present.

How many different amino acids are used to make proteins in living organisms, and what are they called?

There are 20 naturally occurring amino acids that form all proteins in living organisms.
They are referred to as standard amino acids.

Examples include:

• Nonpolar: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline.

• Polar: Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine.

• Acidic: Aspartic acid, Glutamic acid.

• Basic: Lysine, Arginine, Histidine.

Each amino acid has a unique side chain (R group) giving it specific chemical characteristics essential for protein structure and function.

What are zwitterions, and how do amino acids exist as zwitterions?

A zwitterion is a molecule with both positive and negative charges but is overall electrically neutral.

In aqueous solutions around physiological pH (~7):

• The amino group (–NH₂) gains a proton → –NH₃⁺ (positive).

• The carboxyl group (–COOH) loses a proton → –COO⁻ (negative).

This allows amino acids to act as buffers, maintaining pH stability in biological systems.

What is meant by “essential” and “non-essential” amino acids?

• Essential amino acids cannot be synthesized by the human body and must be obtained from the diet. Examples: leucine, lysine, valine, methionine, tryptophan, threonine, phenylalanine, histidine, isoleucine.

• Non-essential amino acids can be synthesized by the body from other compounds, such as alanine or glutamic acid.

If even one essential amino acid is missing, protein synthesis is halted because the genetic code requires all 20 amino acids to produce proteins correctly.