A Level Bio 1.4 Protein & Enzymes

A student investigated starch digestion by mixing starch with a solution of the enzyme used to digest starch. The student did a biochemical test for protein when starch digestion was completed.

The student’s test for protein was positive. Explain why. (2)

(Positive because) enzymes are protein (1)

(because) enzymes not used up (in reactions) (1)

A scientist investigated the hydrolysis of the protein casein.

The scientist:

• mixed a solution of a protease enzyme with a solution of casein

• then measured the casein concentration in the mixture at intervals

• controlled all relevant variables appropriately.

For this investigation, identify:

the independent variable and the dependant variable (2)

(independant) Time (of measurement/test/sample taken) (1)

(Dependant) Casein concentration (1)

Describe the primary structure of all proteins (2)

Sequence/order of amino acids (1)

(Joined by) peptide bonds (1) 

Complete the passage with the appropriate terms

ATP synthase comprises several polypeptides, so is said to have a _______________ structure. It catalyses the synthesis of an ATP molecule by a _______________ reaction; this involves the _______________ of a water molecule. The ATP synthase in the figure above is in a mitochondrion so would catalyse reactions during _______________.

1) Quaternary 

2) Condensation 

3) Release/loss/formation

4) (Aerobic) respiration  

All correct = 2 marks, 2–3 correct = 1 mark,

As shown in the figure above, ATP synthase has two functions.

• It catalyses the synthesis of ATP.

• It allows the movement of H+ ions.

Suggest how the shape of the ATP synthase allows it to have these two

functions.

Explain your answers.(4)

The diagram shows the primary structure of part of a polypeptide. Each shape represents an amino acid. Identical amino acids have the same shape.

Name the type of peptidase which will hydrolyse the bond labelled G in the diagram above. (1) 

Endo(peptidase) (1) 

A scientist used an enzyme to digest a polypeptide containing 101 amino acids.

The digestion produced a range of smaller polypeptides.

The scientist determined the number of amino acids in each of the polypeptides

produced. He also counted the number of polypeptides of each length.

The table below shows some of the scientist’s results.

Use the information in the table above to calculate the number of

polypeptides:

6 amino acids in length ______________________________________

20 amino acids in length ______________________________________ (2) 

(6 amino acids in length) 1 (1) 

(20 amino acids in length) 2 (1) 

Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst. (3)

Substrate binds to the active site/enzyme (1)

Active site changes shape (slightly) so it is complementary to substrate (1)

Reduces activation energy (1)

The scientists investigated the effect of concentration of inorganic phosphate (Pi) on ATP synthase activity.

After 2 minutes, they stopped each reaction and then measured the concentration of ATP.

Suggest and explain a procedure the scientists could have used to stop each reaction (2)

Boil (1)

Denatures the enzyme/ATP synthase (1)

Explain the change in ATP concentration with increasing inorganic phosphate concentration. (2)

(With) increasing Pi concentration, more enzyme-substrate complexes are formed (1) 

At or above 40 (mmol dm-3) enzyme concentration is a limiting factor (1) 

When bread becomes stale, the structure of some of the starch is changed.

This changed starch is called retrograded starch.

Scientists have suggested retrograded starch is a competitive inhibitor of amylase in the small intestine.

Assuming the scientists are correct, suggest how eating stale bread could help to reduce weight gain. (3)

Less hydrolysis of starch (1) 

to maltose (1) 

(so ) less absorption (of glucose) (1) 

Describe how the structure of a protein depends on the amino acids it contains.(5)

Structure is determined by (relative) position of amino acid/R group (1)

Primary structure is sequence/order of amino acids (1) 

Secondary structure formed by hydrogen bonding (between amino acids) (1) 

Tertiary structure formed by interactions (between R groups) (1) 

Quaternary structure contains >1 polypeptide chain (1) 

 Figure 2 shows the SGLT1 polypeptide with NH₂ at one end and COOH at the other end. Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end. (2) 

One amine/NH2 group joins to a carboxyl/COOH group to form a peptide bond (1) 

(so in chain ) there is a free anime group at one end and a free carboxyl group at the other (1) 

Explain how the active site of an enzyme causes a high rate of reaction. (3)

Lowers activation energy (1)

Induced fit causes active site (of enzyme) to change shape (1)

(so) enzyme-substrate complex causes bonds to form/break (1)

Describe a biochemical test to confirm the presence of protein in a solution. (2) 

Add biuret (reagent (1) 

(positive result) lilac (1) 

Describe how a quaternary protein is formed from its monomers. Do not include the process of translation in your answer. (5)

Amino acids joined by peptide bond(s) (1) 

(by) condensation reaction(s) (1) 

Secondary structure is formed by hydrogen bonding (1) 

Tertiary structure formed by interactions (between R groups) (1) 

Quaternary structure contains >1 polypeptides (1)