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# Proteins (polypeptides)

## The peptide bond

Two amino acids can be condensed to form a dimer (called dipeptide): The central group of atoms $-CONH-$ of such a dipeptide is known (incorrectly) as "peptide bond" Thus, alanine and glycine may form the dipeptide ala-gly: Note that in ala-gly, ala has the terminal $NH_2$ group and gly the $COOH$ group. In the dipeptide (different!) gly-ala, the opposite is true!

## Polycondensation

Polycondensation forms polypeptides (proteins), for example the tetrapeptide thr-his-gln-his

## Importance

Proteins are "machines" through which all living things work. During digestion of an animal, they are converted back to the amino acids which are used to reconstruct (part of) the proteins of the animal.

Protein-rich foods