Proteins

Biology · B Form and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Proteins are made of amino acids. Every amino acid has a central (alpha) carbon atom joined to four things: an amine group, a carboxyl group, a hydrogen atom and an R-group. The R-group is the only part that differs between the 20 amino acids.

Amino acids join by condensation. A peptide bond forms between the carboxyl group of one and the amine group of the next, and water is released. A long chain is a polypeptide.

The chain folds into a precise three-dimensional shape, which decides what the protein does. Heat or a wrong pH changes this shape. This is denaturation, and the protein stops working.

Rules to remember

Amino acid: amine group (–NH₂), carboxyl group (–COOH), H atom and R-group, all on the alpha carbon.
The peptide bond joins the C of a carboxyl group to the N of an amine group.
A chain of n amino acids has (n − 1) peptide bonds, and (n − 1) water molecules are released in making it.
Essential amino acids cannot be made by the body and must come from the diet; non-essential ones can be made from other amino acids.
Number of possible chains of n amino acids = 20ⁿ.
Denaturation breaks the weak bonds that hold the folded shape; the peptide bonds and the amino acid sequence stay the same.
High temperature breaks bonds by making the molecule vibrate more; a change in pH alters the charges on R-groups.

Common mistake

Students say denaturation breaks peptide bonds or that the enzyme is killed. Only the folded shape is lost; the chain of amino acids is still whole.

Worked example

How many different tripeptides can be made from the 20 amino acids, and how many peptide bonds does each one contain?

A tripeptide has 3 positions, and each can hold any of 20 amino acids.
Number of sequences = 20³ = 20 × 20 × 20 = 8 000.
Peptide bonds = n − 1 = 3 − 1 = 2.

Answer: 8 000 different tripeptides, each with 2 peptide bonds.

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B1.2 Proteins

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