Amino Acids Classification Quiz

Study Notes

Amino Acids Classification

Amino acids are classified into five groups based on R-groups, according to their polarity and interaction with water.
Non-polar, aliphatic R groups are neutral, hydrophobic, and found in the protein's interior.
Non-polar, aromatic R groups are also neutral, non-polar, and engage in hydrophobic interactions.
Polar amino acids are hydrophilic and are divided into:
- Polar-neutral: more soluble in water.
- Polar acidic: contain carboxyl groups.
- Polar basic: contain amino groups.

Nomenclature and Forms

Amino acids have common names and shorthand notations; three-letter and one-letter abbreviations are widely used.
The first letter of the amino acid name is capitalized; for certain amino acids, the first three letters may include only the first letter capitalized.
α-amino acids are typically written in zwitterion form, representing their internal salt structure.

Acid-Base Properties

Amino acids exist as zwitterions—molecules with both positive and negative charges and a net zero charge.
Carboxyl groups release protons (H+) while amino groups accept protons to create positive charges.
The net charge of an amino acid in solution varies with pH, shifting between zwitterions, positive ions, and negative ions.

Isoelectric Point (pI)

The isoelectric point is the specific pH where an amino acid has a net charge of zero.
Amino acids are not attracted to electric fields at their pI.
Different amino acids have unique isoelectric points depending on their structure and side chains.

Guidelines for Amino Acid Forms

At low pH: all carboxyl and amino groups are protonated (net + charge).
At high pH: all groups are deprotonated (net - charge).
At neutral pH: carboxyl groups are deprotonated; amino groups are protonated (net 0 charge).

pKa Values and Peptide Behavior

Different amino acids have varying pKa values that affect their ionization and charge at specified pH levels.
Proteins and peptides act as zwitterions and have isoelectric points; their solubility changes with pH, affecting precipitation.

Formation of Peptides

Emil Fischer proposed that proteins are chains of amino acids connected by peptide bonds (amide bonds).
The peptide bond forms between the α-carboxyl group of one amino acid and the α-amino group of another.

Writing Peptides

Peptides are depicted from left to right, starting with the N-terminal (free -NH3+) to the C-terminal (free -COO-).
N-terminal and C-terminal designations indicate positions in the amino acid sequence.

Peptide Nomenclature

The IUPAC naming convention replaces amino acid name endings with the suffix "-yl," except for the C-terminal amino acid, which retains its full name.
Example notation for a peptide includes the sequence: Ala-Phe-Ser; abbreviation: AFS.

Description

Test your knowledge on the classification of standard amino acids based on their R-group properties. This quiz focuses on groups defined by polarity and their behavior in biological systems. Understand how these classifications influence protein structure and function.