3.1 Mode of Action of Enzymes

Phenylalanine is one of the nine amino acids necessary to sustain human life. Other molecules can bind directly to the surface of phenylalanine and undergo chemical reactions. The chemical reaction facilitated involves the breakdown of a large molecule into two parts, a process termed hydrolysis.

Which level of protein structure is most important for this hydrolysis reaction?

The graph shows the activation energy of an enzyme-catalysed reaction and the same reaction without a catalyst.

Which of the following statements is not true?

The diagram below shows a metabolic pathway that generates an important hormone: 

A metabolic poison is able to slow the production of the hormone by inhibiting one or more of the enzymes in the pathway.  The effect of adding this poison is a large increase in the amount of reactant, a decrease in the amount of intermediate X, a slight increase in amount of intermediate Y and a large decrease in the amount of the hormone 

Which enzymes is the metabolic poison targeting?

The HIV virus produces a long polypeptide that is hydrolysed by a protease enzyme to producing several smaller peptides. The smaller peptides help the virus to successfully invade their host, so this viral protease is the target of many anti-AIDS drugs.

Which feature is important if this drug is to be successful in preventing AIDS?

This figure shows a calibration curve for of blood samples at 5% concentration with known amounts of glucose.  An average person has a blood glucose level of around 350 mg/l when measured 30 minutes after consuming a starchy meal.  

The blood of a person with type 2 diabetes was sampled 30 minutes after eating a starchy meal.  A colorimeter was used to assess the amount of glucose present in the person’s blood. To do this, first the blood sample was diluted to 1/2^0th of its original concentration with water (so that light could penetrate more easily). Then Benedict’s reagent was added, and the solution was heated in a water bath for five minutes.  The final step was to measure the absorbance of red light by the sample using a colorimeter.

What absorbance value could reflect the diabetic blood sample?

The following equation shows a reversible reaction:

In this reaction, which row best describes what is occurring?

4-Oxalocrotonate tautomerase forms part of a bacterial metabolic pathway that catabolises several small molecules together into a larger molecule.  With a monomer size of just 62 amino acid residues, this enzyme is one of the smallest enzymes known.  However, in solution, the enzyme forms a hexamer of six identical subunits, so the active site is formed by amino acid residues from several subunits.  

Which level of protein structure is most important in the catabolic reaction in the bacterial metabolic pathway?

In an experiment, students were trying to determine the amount of starch in different types of flour.  They blended equal amounts of the different foods in 100 cm3 of water and then added iodine to each of the solutions.  They then took a sample from each of the blended food mixtures and analysed them with a colourimeter.  The colourimeter was set to measure the percentage transmission of dark blue light.  

The graph below shows their results:

Which food substance contained the most starch?

The graph below shows a calibration curve for the absorbance of different concentrations of glucose in solution. 

Calibration curves can be used to determine the concentration of unknown samples.  A student was wanted to determine how much sucrose they had in an unknown sample using the figure above. To make their results comparable, they first added sucrase to hydrolyse the sucrose into glucose and fructose.  They then set their colourimeter to the same settings as the one used in the calibration curve and tested the unknown sample. The sample had an absorbance of 25%.  

How much sucrose was present in the unknown solution?

When investigating the rate of reaction of the enzyme lipase on the hydrolysis of triglycerides, the pH must be maintained to prevent the lipase denaturing.

What is the correct reason for this?

The breakdown of hydrogen peroxide to water and oxygen is catalysed by the enzyme catalase.  In an investigation into the effect of temperature on the rate of reaction of catalase, mashed liver (which contains catalase) was added to hydrogen peroxide.

What would the independent variable be in this experiment?

The graph below shows the activation energy of an enzyme-catalysed reaction, and the same reaction without a catalyst.

What is the value of activation energy for the enzyme-catalysed reaction?

Which levels of protein structure always determine the specificity of an enzyme?

Which of the following statements about enzymes is not correct?

Which type of bonds would be the last to break when an enzyme is heated above its optimum temperature?

Why do large increases in temperature or pH reduce the rate of reaction in enzymes?

Colorimeters are often used in biology to provide quantitative data on the rate of enzyme activity. 

Which of the following statements about colorimeters is not true?   

The graph below shows the activation energy of an enzyme-catalysed reaction and the same reaction without a catalyst.

Which values represent the activation energy of the uncatalysed reaction?