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Protein Denaturation Comprehension

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Protein Denaturation Comprehension

Read the information carefully, then use evidence from it to answer the questions. Denaturation changes a protein’s shape; it does not usually break the peptide bonds linking its amino acids.

Case file: Why did the egg white turn firm?

Isla is testing how heat and acidity change proteins for a food technology investigation. An egg white contains water and proteins, including albumin. In a raw egg white, these proteins are folded into specific three-dimensional shapes. When Isla warms one sample, the proteins gain kinetic energy and begin to unfold. Exposed parts of neighbouring protein molecules form new attractions, creating a linked network that traps water. The egg white changes from clear and runny to opaque and firm. Cooling it does not restore the original liquid egg white: the molecules have formed a tangled network, so this change is effectively irreversible. In a second test, Isla adds lemon juice to a separate sample. The lower pH changes the charges on parts of the proteins and disrupts attractions that help maintain their folded shapes. Proteins can unfold and clump together. Heat and extreme pH can both denature proteins, but the temperature at which a protein changes depends on the particular protein and the conditions. There is no single denaturation temperature for all proteins. An enzyme is also a protein. Its folded shape forms an active site that fits particular reactant molecules. If denaturation changes the active site, the reactants may no longer bind effectively and the reaction rate falls. Denaturation is not the same as breaking the protein’s peptide bonds, which would alter its primary amino-acid sequence. Some proteins can refold if conditions return to normal before they aggregate, but others cannot. In food preparation, denaturation can be useful: cooking firms egg white and changes its texture. In the human body, excessive heat can disrupt proteins and interfere with cell function.

Read, reason and apply

Use the case file to complete each task. For questions requiring explanations, support your answer with a relevant detail.

1.Which statement best defines protein denaturation as described in the case file?

The protein’s amino-acid sequence is replaced with a new sequence.
The protein’s three-dimensional shape changes, which can affect its function.
Amino acids join together to make a brand-new protein.
The protein is broken down completely into amino acids.

2.Select the two conditions in Isla’s investigation that can denature proteins.

Warming the egg-white sample
Adding lemon juice to lower pH
Cooling a sample gently in a refrigerator
Adding water to dilute the sample

3.Put these events in the order that explains how heated egg white becomes firm: (A) exposed protein regions form new attractions; (B) the egg white becomes opaque and firm; (C) proteins gain kinetic energy and unfold; (D) linked proteins form a network that traps water.

4.Why does the case file not give one exact temperature at which all proteins denature?

5.Explain how denaturation can reduce the activity of an enzyme. Include the terms “active site” and “reactants”.

6.True or false? Denaturation necessarily breaks the peptide bonds and changes a protein’s primary amino-acid sequence. Correct the statement if it is false.

TrueFalse

7.Isla cools the firm, cooked egg white back to 20°C. Predict what happens and explain why cooling does not restore the raw egg white.

8.Give one useful food-technology effect and one possible harmful biological effect of denaturation from the case file.

9.A student claims, “Any denatured protein will refold as soon as it cools.” Is this claim supported by the case file? Use evidence to justify your answer.