Chapter 19 · Biochemistry
19.5Proteins
10 min · two checks
Predict
What is the bond that links amino acids in a protein, and what does a protein’s sequence decide?
The idea
- Describe an amino acid and a peptide bond.
- Explain that sequence is information.
An amino acid has a carbon (the α-carbon) attached to an amino group, a carboxylic acid, a hydrogen, and a side chain R. Twenty side chains appear in human proteins: some hydrophobic, some polar, some acidic, some basic. Cells join amino acids by condensation. The link is a peptide bond, chemically an amide. A short chain is a peptide. A long one, folded and functional, is a protein. The order of R groups is the primary structure, and it is what a gene specifies.
There is no generic “protein function.” Enzymes are protein catalysts. Hemoglobin is a protein transporter. Keratin and collagen are structural. Antibodies are recognition proteins. Each works because its particular sequence folds into a surface that fits a particular molecule. Change one amino acid in the wrong place — sickle-cell hemoglobin is the teaching example — and the function changes. Denaturing with heat or acid unfolds the chain without breaking every peptide bond. Cooking an egg is denaturation you can see.
Keep these
- Amino acids: amino group, carboxyl group, distinctive side chain.
- Peptide bonds are amides formed by condensation.
- Function follows fold, and fold follows sequence.