Structure & Functions of Proteins

Biology: IB Diploma Higher Level, topic 5.2.1

Polypeptides

Amino acids are the monomers that join together to form polymers called polypeptides (proteins).

Structure of proteins

Proteins are made up of amino acids.
Dipeptides are formed from the condensation of two amino acids.
Polypeptides are formed by the condensation of many amino acids.

Structure of amino acids

Each amino acid has a central carbon atom (alpha carbon).
There are four atoms or groups of atoms bonded to the central carbon atom:
NH₂ (an amine group).
COOH (a carboxyl group).
H (a hydrogen atom).
R (a side group).

R groups

The R group is different in each amino acid.
The R group determines how the amino acid interacts and bonds with other amino acids in the polypeptide.

Essential amino acids

There are 20 different types of amino acids that are common in all organisms.
10 of these are essential amino acids in humans because the human body cannot produce them and they are obtained from the diet.

Glycine

Glycine is an example of an amino acid.
Glycine has a hydrogen atom in its R group.
Glycine is the only amino acid that does NOT have a carbon atom in its R group.

Peptide Bond

Each amino acid is attached to another amino acid as a result of a condensation reaction. These reactions cause peptide bonds to form.

Polypeptides

Polypeptides are made from chains of amino acids.
There are amino acids at each end of the polypeptide chain.
These amino acids form the two end terminals:
The N-terminal (amine terminal).
The C-terminal (carboxyl terminal).

The peptide bond

When two amino acids react together, a bond forms between the carboxyl group of one amino acid and the amine group of a second amino acid.
One water molecule is released as a by-product.
The bond formed between two amino acids is a covalent bond called a peptide bond.

Function of Proteins

Proteins have a diverse range of functions. Each cell in a living system may contain thousands of proteins, each with a unique structure and function. Functions of proteins include:

Enzymes

Enzymes are biological catalysts.
Enzymes increase the rate of a reaction without being used up in the reaction.
The enzymes are usually tightly folded, complex proteins that are soluble.

Antibodies

Antibodies are made up of polypeptide chains.
Antibodies are used in the immune response.
Antibodies are diverse proteins.
Each antibody has a different sequence of amino acids.

Transport proteins

Transport proteins include channel proteins.
Channel proteins transport molecules across the cell membrane.
Channel proteins transport molecules that are too large to diffuse freely or molecules that carry a charge.

Structural proteins

Structural proteins are long, strong polypeptide chains.
Structural proteins are connected by cross-links that hold the chains parallel to each other.
E.g. Collagen and keratin.

Test yourself on Structure & Functions of Proteins

What monomers are bonded together to form polypeptides?
What are the four groups surrounding the central carbon atom in an amino acid?
How many different amino acids are there?
Dipeptides are formed by what reaction?
Which statements are true about the polypeptide chain?

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