Proteins: Enzymes (AQA A Level Biology): Flashcards

The specificity of an enzyme is determined by the complementary shape of its active site to the substrate, which results from the enzyme's unique tertiary structure.

The shape of an enzyme's active site is determined by the tertiary structure of the protein.

Extreme heat or pH can change the shape of the enzyme's active site, causing denaturation, which prevents substrate binding.

Activation energy is the minimum amount of energy that substrates must have for a chemical reaction to occur and products to be formed.

Enzymes catalyse reactions by lowering the activation energy required.

False.

The enzyme-substrate complex is temporary, and the enzyme is released after the reaction to be used again.

The induced-fit model (or induced-fit hypothesis) describes how an enzyme’s active site changes shape to achieve an ideal fit with its substrate, allowing efficient catalysis.

Conformational changes are slight changes in the shape of an enzyme’s active site or the substrate that occur as the substrate enters the enzyme, helping to achieve ideal binding.

The induced-fit model explains that an enzyme’s active site changes shape to ensure an ideal binding arrangement with the substrate.

The enzyme changes shape to form an ideal binding arrangement with the substrate, which maximises its ability to catalyse the reaction.

The lock and key model proposes that an enzyme’s active site has a rigid shape that is a precise fit for a specific substrate, like a key fitting a lock.

False.

Proteins are not rigid structures. Experiments show that enzymes can change shape, especially when binding to substrates.

Experiments using X-ray diffraction techniques produced 3D images of enzymes and showed that the active site of the enzyme changed after binding to the substrate.

New techniques such as X-ray diffraction allowed scientists to observe that enzyme active sites change shape when binding substrates, supporting the induced-fit model.

Catalase is an enzyme found in most organisms that breaks down hydrogen peroxide into water and oxygen.

The volume of oxygen generated over a set time is measured to determine the rate of product formation in a catalase-catalysed reaction.

Hydrogen peroxide is a common but toxic by-product of metabolism that is broken down by catalase.

False.

Catalase breaks down hydrogen peroxide into water and oxygen.

Amylase is a digestive enzyme that hydrolyses starch into maltose.

Iodine in potassium iodide solution turns blue-black when starch is present.

Amylase functions best at pH 7 and 37 °C.

A colorimeter can measure the decrease in absorbance or increase in transmission as the starch is broken down, allowing the rate of reaction to be determined.

A calibration graph is plotted with starch concentration on the X-axis and percentage absorbance or transmission on the Y-axis.

The independent variable is the variable that is intentionally changed to investigate its effect on the rate of an enzyme-catalysed reaction.

True.

All other variables are control variables and must be kept constant to ensure valid results.

An enzyme rate experiment is an experiment carried out to determine the effect of changing a particular factor on the rate of a reaction catalysed by an enzyme.

The factors that can be changed include temperature, pH, enzyme concentration and substrate concentration.

The rate of an enzyme-catalysed reaction can be measured by determining how much product is made or how much substrate is broken down in a given time period.

A line graph is a type of graph used to present the results of enzyme rate experiments, plotting the independent variable on the x-axis and the dependent variable on the y-axis.

A line of best fit is drawn to identify trends in the data and to allow interpolation or extrapolation of values.

When plotting a line graph, the independent variable is placed on the x-axis and the dependent variable is placed on the y-axis.

False.

A line of best fit should only go through the origin if the data and trend allow it, such as when the rate is zero at zero concentration.

Two important tips are to plot data points accurately and label axes with units included.

A tangent is a straight line that touches a curve at one point without crossing it, used to find the gradient (rate of change) at that specific point on a graph.

The gradient of a straight line on a reaction rate graph represents the rate of change of the reaction, which is constant throughout the graph.

Enzyme-catalysed reaction graphs are often curved because the rate of reaction changes over time, resulting in an ever-changing gradient.

To find the initial rate of reaction on a curved graph, draw a tangent that just touches the curve at time = 0.

The initial rate of reaction is the rate of reaction at the very start of the experiment, when time = 0.

To calculate the gradient, divide the change in y-axis (rise) by the change in x-axis (run).

False.

A tangent can be drawn at any point on the curve to calculate the rate of reaction at that specific time.

The phrase “rise over run” helps you remember how to calculate a gradient.

To draw a tangent accurately, use a ruler and pencil to make a perfectly straight line that just touches the curve at the chosen point.

The optimum temperature for an enzyme is the specific temperature at which it catalyses a reaction at the maximum rate.

At lower temperatures, enzyme activity decreases because molecules move more slowly, leading to a lower frequency of successful collisions and less frequent enzyme-substrate complex formation.

At high temperatures, enzyme molecules move more quickly, increasing the frequency of successful collisions.

False.

At high temperatures, enzymes begin to denature, which damages the active site and sharply decreases the reaction rate.

Denaturation is when the bonds holding the enzyme's shape break, causing the tertiary structure and active site to change so the substrate can no longer bind.

If the substrate can no longer bind to the active site, the enzyme is said to be denatured.

Very few human enzymes function above 50°C because high temperatures break hydrogen bonds, causing the enzyme to denature. Human body temperature is about 37°C, so even temperatures above 40°C can cause denaturation.

At low temperatures, molecules move more slowly, resulting in fewer successful collisions between enzymes and substrates and less frequent enzyme-substrate complex formation.

The optimum pH is the specific pH at which an enzyme works at its maximum rate of activity.

At extremes of pH, enzymes can be denatured, losing their functional shape and activity.

All enzymes have an optimum pH, at which their activity is highest.

In enzymes, denaturation is the irreversible loss of structure, usually due to extreme pH or temperature, causing the active site to no longer function.

The tertiary structure of an enzyme is held together by hydrogen and ionic bonds.

A change in pH can cause hydrogen and ionic bonds to break in enzymes.

A change in pH can alter the shape of the active site, making it more difficult or impossible for the enzyme to bind its substrate.

False.

Pepsin’s optimum pH is 2, which matches the acidic environment of the stomach.

A buffer solution is a solution that maintains a constant pH even when acids or bases are added, useful for studying enzyme activity at different pH values.

pH can be calculated using the formula: pH = -log₁₀ [H⁺].

Enzyme concentration refers to the amount of enzyme molecules present in a reaction mixture, which determines the number of active sites available for substrate binding.

If there is sufficient substrate, increasing the enzyme concentration increases the initial rate of reaction linearly because more active sites are available for enzyme-substrate complex formation.

False.

If substrate is limited, further increases in enzyme concentration will not increase the reaction rate because the amount of substrate becomes a limiting factor.

The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation.

As long as there is sufficient substrate available, the initial rate of reaction increases linearly with enzyme concentration.

An enzyme-substrate complex is the temporary structure formed when an enzyme binds to its substrate at the active site.

The rate of reaction increases when more active sites are available because there is a greater chance for substrate molecules to bind and form enzyme-substrate complexes, leading to more product being formed per unit time.

A limiting factor is a factor that, when in limited supply, restricts the rate of reaction, so that increasing other factors has no further effect on the rate.

When substrate is limited, there is not enough substrate to occupy all the available active sites.

Adding more enzyme provides extra active sites that remain unused, so substrate becomes the limiting factor and the rate does not increase further.

If the amount of substrate is limited, further increases in enzyme concentration will not increase the reaction rate, as the amount of substrate becomes a limiting factor.

It shows a linear (straight-line) increase, because the rate of reaction rises in proportion to enzyme concentration as more active sites become available for enzyme-substrate complex formation.

Substrate concentration refers to the amount of substrate molecules present in a solution available to bind with enzymes at any given time.

An enzyme-substrate complex is a temporary association where an enzyme binds to its substrate at the active site.

Increasing the substrate concentration increases the rate of reaction up to a certain point, but once all enzyme active sites are saturated, the rate will no longer increase with added substrate.

Vmax is the maximum rate of an enzyme-catalysed reaction when all active sites are occupied by substrate and the enzyme is working at full capacity.

False.

Once all the enzyme active sites are saturated, further increases in substrate concentration do not increase the rate of reaction.

As the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases.

When all enzyme active sites are occupied, any additional substrate molecules have nowhere to bind.

At high substrate concentrations, all enzyme active sites are occupied, so the reaction rate reaches a maximum and cannot increase further even if more substrate is added.

An enzyme inhibitor is a substance that temporarily reduces or stops an enzyme's activity by binding to it.

A competitive inhibitor is a molecule with a similar shape to the substrate that competes for the active site of an enzyme.

A non-competitive inhibitor binds to an enzyme at a site other than the active site, altering the enzyme's shape and preventing substrate binding.

When a non-competitive inhibitor binds to an enzyme, it alters the shape of the enzyme's active site, preventing the substrate from binding.

Competitive inhibitors reduce enzyme activity by competing with the substrate for the active site, temporarily blocking substrate binding.

Competitive inhibitors have a similar shape to the substrate and compete for the active site.

Non-competitive inhibitors bind to an alternative site and change the shape of the active site.

End-product inhibition is a process in which the final product of a metabolic pathway acts as a reversible, non-competitive inhibitor of an enzyme earlier in the pathway, regulating the pathway.

End-product inhibition ensures that metabolic reactions are tightly controlled and balanced, preventing the overproduction of products by slowing or stopping enzyme activity when enough product is present.

In end-product inhibition, as the product level falls, the enzyme returns to its active state and catalyses the reaction again.

Increasing the concentration of an inhibitor reduces the rate of reaction, and if increased enough, can stop the reaction completely.

False.

Increasing substrate concentration cannot overcome non-competitive inhibition because the active site remains altered and unavailable for substrate binding.

For competitive inhibitors, increasing the substrate concentration can increase the rate of reaction again.

An independent variable is the variable that is deliberately changed in an experiment to observe its effect on the outcome.

A control variable is any variable that must be kept constant in an experiment to ensure a fair comparison.

Changing only one variable at a time ensures that any change in the rate of reaction can be attributed to that specific variable, making the results more reliable.

In enzyme rate experiments, the variable that is changed is called the independent variable, and those that are kept the same are called control variables.

Two examples of factors that can be changed in an enzyme rate experiment are temperature and pH.

True.

If control variables are not kept constant, they may affect the outcome, making experimental results unreliable.

Uncertainty is the amount of error your measurements might contain due to limitations in the sensitivity of the apparatus.

The formula is: percentage error = (uncertainty ÷ measured value) × 100.

A gas syringe measures volumes to the nearest 1 cm^³. The uncertainty is ± 0.5 cm^³.