Enzymes

Catalysts and Enzymes

Every living cell carries out hundreds of chemical reactions at once. Enzymes make those reactions fast enough for life.

Catalysts

A catalyst makes a reaction go faster, and it is the same at the end as at the start, so a small amount can be used again and again.

Enzymes in living organisms

Enzymes are proteins. They are involved in all metabolic reactions, the chemical reactions that take place in living organisms, where they act as biological catalysts. A cell makes many different enzymes.

Why life needs enzymes

At the temperature of a living body, many chemical reactions would be too slow to keep an organism alive. Enzymes increase the reaction rate to the level necessary to sustain life. Respiration, building proteins and digesting food all depend on enzymes.

Some enzymes and their substrates

Worked example (comparing with and without an enzyme): A reaction takes 120 s without an enzyme and 6 s with it. How many times faster is it with the enzyme? Step 1. Rate is the inverse of the time taken, so less time means a faster reaction. Step 2. With the enzyme: 120 ÷ 6 = 20. Step 3. The enzyme is not changed, so the same enzyme could speed up more of the reaction. Answer: 20 times faster.

Enzyme Action

Each enzyme has a region with a particular shape. A substrate whose shape matches it can bind there, and the reaction takes place.

How an enzyme works

The enzyme has an active site whose shape is complementary to the shape of its substrate. The substrate fits into the active site, the reaction takes place, and the products form and leave. The enzyme is unchanged and can take another substrate.

ExtendedEnzyme-substrate complex and specificity

When the substrate binds, it forms an enzyme-substrate complex. Because the shape and fit of the active site match only a certain substrate, each enzyme is specific: it catalyses one reaction, and a substance with a different shape does not fit.

Worked example (predicting whether an enzyme will work): Enzyme X has an active site complementary to substrate A. Substrate B has a different shape. Will enzyme X act on B? Step 1. The enzyme acts only on a substrate whose shape fits the active site. Step 2. B has a different shape, so it does not fit. Step 3. It does not form an enzyme-substrate complex, so enzyme X does not act on B: enzymes are specific. Answer: No: B does not fit.

Temperature and Enzymes

Temperature changes enzyme activity in a pattern you must be able to describe from a graph.

Describing the pattern

As temperature rises towards the optimum temperature the rate of reaction increases, and at the optimum the enzyme is most active. Above the optimum the rate falls quickly, because the enzyme is denatured: its shape changes permanently and it stops working. Once denatured, an enzyme does not recover when it cools.

ExtendedExplaining the pattern

As temperature rises the molecules gain kinetic energy and move faster, so there are more frequent effective collisions between enzyme and substrate. Above the optimum, the enzyme’s shape changes, including the active site, so the shape and fit with the substrate is lost: denaturation.

Worked example (reading the graph): Using Fig 5.2, state the optimum temperature and describe what happens to the rate between 40 °C and 60 °C. Step 1. The peak of the curve is at 40 °C. Step 2. From 40 °C to 60 °C the rate falls, reaching zero near 60 °C. Step 3. The enzyme is denatured above the optimum, so it stops working. Answer: Optimum 40 °C; rate falls to zero.

pH and Enzymes

Every enzyme has an optimum pH. Different enzymes have different optima, which suits the part of the body where each one works.

Effect of pH

An enzyme is most active at its optimum pH. At a pH that is too high or too low, activity falls, and an extreme pH denatures the enzyme. The enzyme in the mouth, amylase, works best near neutral pH. Pepsin, a protease in the stomach, works best in acid.

ExtendedExplaining the effect of pH

A change in pH changes the shape of the enzyme, including the active site. The shape and fit with the substrate is lost, and the enzyme is denatured.

Worked example (predicting from pH): Amylase works best near pH 7. Predict what happens to its activity if it is mixed with stomach acid at about pH 2. Step 1. pH 2 is far from the optimum for amylase, so this is a strongly acid pH for this enzyme. Step 2. An extreme pH denatures the enzyme. Step 3. The activity falls and stops. Answer: Activity stops.

Investigating Enzyme Activity

Two standard investigations measure how fast an enzyme works while one condition, temperature or pH, is changed.

Amylase and starch

Warm the amylase and the starch solution separately in a water bath at a set temperature, then mix them. Every 30 seconds, take a drop of the mixture onto a spotting tile and add a drop of iodine solution. While starch remains, the drop turns blue-black. Record the time at which the iodine stops turning blue-black and stays yellow-brown; the starch has then been broken down.

Catalase and hydrogen peroxide

Add catalase, for example from liver or potato, to hydrogen peroxide. Oxygen is released, so measure the volume of gas collected in a set time, or, with a drop of washing-up liquid added, the height of foam formed.

To investigate pH, change the pH with buffer solutions and keep the temperature the same. Repeat each condition and calculate a mean.

Worked example (choosing the variables): A student times how long amylase takes to digest starch at 20, 30, 40, 50 and 60 °C. Name the independent and dependent variables. Step 1. The factor the student changes is temperature. Step 2. The factor measured is the time taken for the starch to be digested. Step 3. So temperature is independent and time is dependent. Answer: Independent: temperature.

Reading Results

Results from enzyme investigations are often times. A shorter time means a faster rate, so convert times into rates before comparing.

Worked data

The shortest time, 50 s, is at 40 °C, so 40 °C is the optimum of the temperatures tested. At 60 °C the starch is not digested at all, because the enzyme has been denatured.

Describing a trend

A full description gives the direction, the values and the turning point. For the table: the rate increases from 0.005 per s at 20 °C to 0.020 per s at 40 °C, then decreases to 0.008 per s at 50 °C and to zero at 60 °C.

Controls

A control tube contains boiled enzyme in place of active enzyme. If the starch is still there at the end, it was not broken down without active enzyme, so the working enzyme caused the change in the other tubes. Using the same amount of everything else keeps the test fair.

Worked example (calculating and comparing rates): Use the table to find the rate at 30 °C and say how many times faster it is than at 20 °C. Step 1. Rate at 30 °C = 1 ÷ 100 = 0.010 per s. Step 2. Rate at 20 °C = 1 ÷ 200 = 0.005 per s. Step 3. 0.010 ÷ 0.005 = 2, so the rate doubles. Answer: 0.010 per s; twice as fast.