Biology 9700/13 — May/June 2010

Background Concept

Lysozyme is an enzyme found in tears, saliva and other body secretions. Like all enzymes, it is a protein — a long chain of amino acids joined by peptide bonds. Enzyme cleaners and biological washing powders therefore often contain another class of enzymes, proteases, which hydrolyse the peptide bonds of proteins, breaking them into shorter peptides and free amino acids that dissolve in water and wash away.

Enzymes are highly specific because of the three-dimensional shape of their active site. A protease has an active site complementary to peptide bonds, so it binds to a protein substrate and catalyses the hydrolysis of those bonds. This is the same lock-and-key / induced-fit principle that makes lysozyme specifically hydrolyse the β-1,4-glycosidic bonds in bacterial cell wall peptidoglycan.

Understanding the Question

The question states that deposits of lysozyme form on contact lenses. The key biological point to identify is the chemical nature of lysozyme — it is a protein. The cleaner must therefore contain an ingredient that breaks down proteins. The command word is implicit ("Which ingredient would be effective…"), asking the candidate to select the option that actually degrades the deposit.

Approach

1. Identify what lysozyme is made of → protein.
2. Ask what removes/digests proteins → proteases.
3. Eliminate the other options because they either are not enzymes, do not target proteins, or are not ingredients at all.

Step-by-Step Reasoning

• Option A — ethanol: a small polar organic solvent. It dissolves many non-polar substances but is not an enzyme and does not hydrolyse peptide bonds. It would also be irritating to the eye. Not effective for protein removal. ✗
• Option B — lysosomes: these are membrane-bound organelles inside cells that contain hydrolytic enzymes (including proteases). They are not a free ingredient and cannot be added to a cleaning solution intact. ✗
• Option C — pH buffers: these resist changes in pH and are useful for stabilising the solution, but they do not chemically break down proteins. (Extreme pH can denature proteins, but a buffer is by definition the opposite of an extreme pH condition.) ✗
• Option D — proteases: enzymes that specifically hydrolyse peptide bonds in proteins. Lysozyme, being a protein, is an ideal substrate for a protease. The proteolytic breakdown products are soluble peptides and amino acids that can be washed off the lens. ✓

Key Takeaways

• Lysozyme is a protein, so it can be removed by a protease (an enzyme that digests protein).
• This is the same principle behind biological washing powders, which use proteases, lipases and amylases to remove protein, fat and starch stains respectively.
• When a question describes a "deposit" or "stain", identify its chemical nature first (protein, lipid, starch, etc.) and then choose the appropriate hydrolytic enzyme.

Common Mistakes

• Confusing lysosomes with lysozyme: they sound similar but are very different — lysozyme is a single enzyme, lysosomes are cellular organelles. Lysosomes cannot be put into a cleaner.
• Choosing pH buffers because they "stabilise" the cleaner — buffers resist pH change but do not degrade the deposit.
• Choosing ethanol because it is a familiar disinfectant — ethanol denatures some proteins but is not used to digest them away, and it is unsuitable in an eye-care product.

Things to Be Careful About

• Always read what the deposit is made of; the chemistry of the deposit dictates the chemistry of the remover.
• A "buffer" must not be confused with an acid or alkali that denatures protein — a buffer specifically minimises pH change.
• "Lysosome" vs "lysozyme" is a classic CIE distractor pair; remember the -zyme suffix denotes an individual enzyme.