Proteins

Proteins are natural polymers found in all living things. They are made from many small molecules called amino acids joined in long chains. Because each join is an amide link, proteins are called natural polyamides.

Amino acid monomers

All amino acids share a general structure: $\mathrm{H_2N{-}CHR{-}COOH}$. The $\mathrm{H_2N{-}}$ is the amino group, $\mathrm{{-}COOH}$ is the carboxyl (acid) group, and R is the side chain that can change. For example, $\mathrm{R=H}$ in glycine; $\mathrm{R=CH_3}$ in alanine.

How proteins form: condensation polymerisation

Amino acids join when the carboxyl group of one reacts with the amino group of another. A molecule of water is removed and a peptide (amide) bond forms.

$$\mathrm{{-}COOH + H_2N{-} \; \longrightarrow\; {-}CO{-}NH{-} + H_2O}$$

Two amino acids make a dipeptide; many make a polypeptide (protein). The repeating linkage in the chain is the peptide bond: $\mathrm{{-}CO{-}NH{-}}$.

Recognising the peptide (amide) bond

Look for a carbonyl $\mathrm{C{=}O}$ directly joined to a nitrogen $\mathrm{N}$: $\mathrm{{-}CO{-}NH{-}}$. This is different from an ester link $\mathrm{{-}CO{-}O{-}}$.

Breaking proteins: hydrolysis

Proteins can be broken back into amino acids by adding water (hydrolysis), usually with acid, alkali, or enzymes.

$$\mathrm{{-}CO{-}NH{-} + H_2O \; \longrightarrow\; {-}COOH + H_2N{-}}$$

Real-world connections

Proteins in food supply amino acids for growth and repair. The sequence of R groups makes each protein fold into a specific shape, giving roles like enzymes, muscle, and hair. Unlike many plastics, proteins are biodegradable.

Common misconceptions

• Proteins are not only from animals; plants have proteins too.
• A peptide bond is an amide ($\mathrm{CONH}$), not an ester ($\mathrm{COO}$).
• R is part of each amino acid, not something added later.
• Condensation removes one water per bond formed, not per atom joined.