1. Protein Structure & Amino Acid Classification

25 vocabulary flashcards covering protein basics, amino-acid classifications by side-chain chemistry, polarity, nutritional requirement, and catabolic fate.

Protein

A biological macromolecule composed of one or more chains of amino acids folded into a specific 3-D shape to perform cellular functions.

Amino Acid

The basic building block of proteins, consisting of an α-carbon attached to an amino group, carboxyl group, hydrogen, and variable side chain (R-group).

R-group (Side Chain)

The variable chemical group on an amino acid that determines its structure, polarity, and chemical behavior.

Polarity (in amino acids)

A measure of how the side chain interacts with water; used to classify amino acids as non-polar, polar uncharged, or polar charged.

Aliphatic Amino Acids

Amino acids whose side chains are straight or branched hydrocarbons; includes Glycine, Alanine, Valine, Leucine, and Isoleucine.

Branched-Chain Amino Acids (BCAAs)

Valine, Leucine, and Isoleucine—aliphatic amino acids with branched hydrocarbon side chains.

Hydroxyl-Containing Amino Acids

Serine, Threonine, and Tyrosine, whose side chains possess an –OH group making them polar.

Sulfur-Containing Amino Acids

Methionine (thioether side chain) and Cysteine (sulfhydryl side chain).

Cystine

A dimer formed when two cysteine residues oxidize to create a disulfide bond (–S–S–), stabilizing protein structure.

Acidic Amino Acids

Aspartic acid (D) and Glutamic acid (E); possess a second carboxyl group giving the side chain a negative charge at physiological pH.

Basic Amino Acids

Lysine (K), Arginine (R), and Histidine (H); contain extra amino/imidazole groups giving a positive charge at physiological pH.

Aromatic Amino Acids

Phenylalanine, Tyrosine, and Tryptophan; contain ring structures that absorb UV light.

Imino Acid

Proline; has a pyrrolidine ring in which the side chain is bonded to the amino nitrogen, restricting backbone flexibility.

Non-Polar (Hydrophobic) Amino Acids

Gly, Ala, Val, Leu, Ile, Met, Phe, and Pro; side chains are hydrophobic and uncharged.

Polar Uncharged Amino Acids

Ser, Thr, Cys, Gln, Asn, and Tyr; contain polar groups but carry no net charge at physiological pH.

Polar Positively Charged Amino Acids

Lys, Arg, and His; side chains are basic and positively charged at physiological pH.

Polar Negatively Charged Amino Acids

Asp and Glu; side chains are acidic and negatively charged at physiological pH.

Essential Amino Acids

Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine – cannot be synthesized de novo (mnemonic: PVT WIM HRLK).

Non-Essential Amino Acids

Glycine, Alanine, Serine, Cysteine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Tyrosine, Proline – synthesized by the human body.

Conditional Amino Acid

Arginine is essential for infants (growth phase) but non-essential for healthy adults.

Glucogenic Amino Acids

Glycine, Alanine, Methionine, Aspartic acid; catabolism yields intermediates for gluconeogenesis (glucose formation).

Ketogenic Amino Acids

Leucine and Lysine; degradation produces ketone-body precursors or fatty acid synthesis intermediates.

Glucogenic & Ketogenic Amino Acids

Isoleucine, Phenylalanine, Tryptophan, and Tyrosine; catabolism yields precursors for both glucose and ketone bodies.

Catabolic Classification of Amino Acids

Grouping amino acids as glucogenic, ketogenic, or both, based on the metabolic fate of their carbon skeletons.

PDB (Protein Data Bank) / PDB-101

A public archive of 3-D structural data for proteins and nucleic acids; PDB-101 provides educational resources such as the video “What is a Protein?”