Proteins

Amino acids and peptide bond formation

Content standard 4.3 covers amino acids as the building blocks (monomers) of proteins. Two amino acids join through a condensation reaction, in which a water molecule is released and a peptide bond forms between them, producing a dipeptide.

Many amino acids joined in a chain form a polypeptide, and one or more polypeptides folded into a functional shape form a protein. The reverse reaction, hydrolysis, breaks peptide bonds by adding water, releasing amino acids, this happens during protein digestion.

Levels of protein structure

• Primary structure, the specific sequence of amino acids in a polypeptide chain.
• Secondary structure, the chain folds into a regular shape, such as an alpha helix or a beta-pleated sheet, held by hydrogen bonds.
• Tertiary structure, further folding produces the overall three-dimensional shape of a single polypeptide, giving it a specific function.
• Quaternary structure, two or more polypeptide chains combine to form one functional protein, such as haemoglobin.

Functions of proteins

Testing for protein and how it is examined

The Biuret test detects protein: when Biuret reagent (or a mixture of dilute sodium hydroxide and dilute copper(II) sulfate) is added to a sample, a purple (violet) colour confirms the presence of protein, while the mixture stays blue if protein is absent. Exam questions commonly ask you to describe how a polypeptide or protein forms from amino acids, to name a level of protein structure from a description, to match a protein to its function, or to state the expected result of the Biuret test.

Why protein shape determines protein function

A protein's final three-dimensional shape decides exactly what it can do, which is why the levels of structure matter beyond simply being memorised terms. An enzyme's tertiary shape creates an active site that fits only its own substrate; if that shape is disrupted, as happens during denaturation by heat or extreme pH, the protein loses its function even though its amino acid sequence is unchanged.

Fibrous proteins such as collagen and keratin, by contrast, form long strands rather than a compact folded shape, which gives them the tensile strength needed for skin, tendons, hair and nails rather than a catalytic or transport role.

Worked exam-style question

Question. Diagram X shows several amino acid molecules joining together. (a) Name the type of reaction shown, and name the small molecule released.

(b) Name the bond formed between two amino acids. (c) A student states that 'a protein is just a very long chain of amino acids'.

Explain why this statement is incomplete. (d) Haemoglobin is made of four separate polypeptide chains combined into one functional molecule.

Name the level of protein structure this represents.

Model answer. (a) Condensation (condensation polymerisation); the small molecule released is water. (b) A peptide bond.

(c) The statement is incomplete because a functional protein is not just a chain (the primary structure), the chain must fold into a specific three-dimensional shape (secondary and tertiary structure) before it can carry out a biological function; an unfolded chain of amino acids cannot act as an enzyme, hormone or antibody. (d) Quaternary structure, because it involves two or more separate polypeptide chains combining into one functional protein.

Practice question

Try this. A food sample is tested with Biuret reagent and turns purple. A second sample of the same food is heated strongly before the same test is repeated, and this time the mixture stays blue.

Explain this difference.

Exam tip

Key terms

These related terms often appear alongside proteins in SPM Biology questions:

• Protein, one or more polypeptide chains folded into a functional three-dimensional shape.
• Amino acid, the monomer (building block) that joins by peptide bonds to form proteins.
• Enzyme, a protein that acts as a biological catalyst, one of the many functional roles proteins carry out.
• DNA, the molecule that carries the genetic code determining the sequence of amino acids in every protein a cell makes.

Source:SRC-DSKP-EN

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