IB Biology Topic B1.2.6: Chemical Diversity in the R-groups of Amino Acids Notes

R-Groups: The Source of Chemical Diversity in Amino Acids

1. All 20 amino acids share the same backbone: a central carbon bonded to an amino group, a carboxyl group, and a hydrogen atom.
2. The fourth bond attaches to the R-group (side chain), which is different for each amino acid.
3. The R-group sets the amino acid's chemical properties, so it decides how the amino acid behaves in a protein.

Non-Polar R-Groups: Hydrophobic and Water-Repelling

1. Some R-groups are made mainly of carbon and hydrogen, making them non-polar and hydrophobic.
   1. They do not form hydrogen bonds with water.
   2. Examples include valine, leucine, and isoleucine.
2. Because they repel water, hydrophobic R-groups can also anchor a protein inside the lipid interior of a membrane.

Polar Uncharged R-Groups: Neutral but Water-Loving

1. Other R-groups contain oxygen or nitrogen atoms that create an uneven spread of charge, making them polar.
   1. They are neutral overall but carry partial positive and negative charges.
   2. These partial charges let them form hydrogen bonds with water and other polar molecules.
   3. Examples include serine, threonine, and asparagine.

Charged R-Groups: Full Positive or Negative Charge

1. Some R-groups carry a full electrical charge at physiological pH.
2. Acidic R-groups lose a proton from a carboxyl group, giving them a negative charge.
   1. Examples are aspartate and glutamate.
3. Basic R-groups gain a proton on an amine group, giving them a positive charge.
   1. Examples are lysine, arginine, and histidine.

R-Group Chemistry Sets What a Protein Can Do

1. The mix of R-group properties along the chain determines how a protein folds, a link explored in later topics.
2. R-group chemistry also decides what an enzyme's active site can bind.
   1. Hexokinase uses polar and charged R-groups in its active site to bind glucose and ATP in the first step of glycolysis.