Proteins as Macromolecules

Proteins

Proteins are macromolecules composed of carbon, hydrogen, oxygen, and nitrogen.

Monomers and Polymers

Proteins, like fats and carbohydrates, are made up of basic subunits called monomers, which build up to polymers. In proteins, the monomer is the amino acid.

Amino Acids

An amino acid consists of:

  • An amine group: A nitrogen atom with two hydrogen atoms attached (NH_2).
  • A carboxyl group: A carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (COOH). Also termed carboxylic acid.
  • A functional group: This is the distinguishing feature of each amino acid. Every amino acid has the core structure of the amine, carbon and carboxyl but the functional group differs.

The combination of the amine group and the carboxylic acid gives amino acids their name.

Functional Groups

The functional group is what differentiates the 20 different types of amino acids. Some amino acids are essential, meaning the body cannot produce them, and they must be obtained through diet. The remaining amino acids can be synthesized by the body.

Functional groups have unique properties:

  • Hydrophobic: Repels water.
  • Acidic
  • Basic
  • Positive charge
  • Negative charge
  • Hydrophilic: Attracts water.

Polypeptide Formation

When amino acids combine, they form a polypeptide, which is a polymer or chain of amino acids. This process involves dehydration, where a water molecule is removed.

  • The hydroxyl (OH) group from the carboxyl end of one amino acid and a hydrogen (H) from the amine side of another amino acid are removed to form water (H_2O).
  • This process links the two amino acids together, and it continues to create a polypeptide.

Protein Structure

A long polypeptide chain starts to resemble a protein, but it is not yet a fully functional protein. The functional groups on the polypeptide chain cause it to fold in upon itself.

Folding

The interactions of the functional groups dictate how the polypeptide folds:

  • Positive and negative functional groups attract.
  • Like-charged functional groups repel.
  • Hydrophilic groups orient towards the liquid environment.
  • Hydrophobic groups embed within the protein away from water.
  • Acids and bases also influence folding.

This folding results in a three-dimensional structure, called the quaternary structure, allowing the folded polypeptide (or group of polypeptides) to become a protein.

Protein Functions

Folded proteins perform various functions in the body. The human body can make around 20,000 to 50,000 different proteins from our genetic material.

Proteins:

  • Act as enzymes that accelerate reactions.
  • Shuttle molecules by transporting them.
  • Serve as receptors that bind to other substances.

The functional groups dictate the folding, which determines the protein's functional properties. A change in even a single amino acid (e.g., from acidic to basic) can alter the protein's folding and function. This is one way that mutations in DNA can affect protein function.