life science

What are enzymes? Why are enzymes sensitive to temperature and pH changes? Are enzymes affected or used up by the reactions they catalyse?

Enzymes are proteins that function as biological catalysts — they speed up biochemical reactions and remain unchanged at the end of the reaction. They are sensitive to temperature and pH because they are proteins; they work efficiently only within an optimal range, and high temperatures or extreme pH denature and inactivate them (enzymes are more sensitive to pH change than to temperature change). Enzymes are not affected by the reactions they catalyse and are not used up, so they’re needed only in small amounts — the same enzyme molecule can process many substrate molecules.

Why are enzymes described as “highly specific”? What must happen before an enzyme-catalysed reaction can occur? Write the general word equation for an enzyme-catalysed reaction.

Each enzyme has a specific shape with a special region called the active site, which can bind only to a specific substrate (complementary shape). The enzyme and substrate must bind to form an enzyme-substrate complex before the reaction can produce products. General equation: Enzyme + Substrate → Enzyme-substrate complex → Products + Enzyme.

What is the lock-and-key hypothesis? What happens to the rate of reaction as temperature rises toward the optimum? What is the optimum temperature?

The substrate molecule (the “key”) has a shape complementary to the enzyme’s active site (the “lock”); the substrate binds to the active site to form an enzyme-substrate complex, and after the reaction, products leave and the enzyme is free to bind another substrate. As temperature rises toward the optimum, the rate increases because enzyme and substrate molecules collide more frequently, increasing the chance of forming an enzyme-substrate complex — the rate roughly doubles for every 10°C rise until the optimum is reached. The optimum temperature is the temperature at which an enzyme catalyses a reaction at its maximum rate (rate of enzyme-substrate complex formation is highest); most human enzymes have an optimum of about 37°C.

What happens beyond the optimum temperature? How does pH affect enzyme activity? Give the optimum pH for pepsin and for trypsin.

Beyond the optimum temperature, the rate of reaction decreases sharply — the enzyme becomes denatured, the active site’s shape changes, the substrate can no longer fit, and denaturation may be irreversible. Each enzyme functions optimally at a particular pH, so a slight change in pH affects the reaction rate; at very high or low pH, ionic bonds within the enzyme are disrupted, changing the active site’s shape so the substrate can’t bind (the enzyme is denatured). Pepsin’s optimum pH is around 2 (acidic, stomach enzyme); trypsin’s optimum pH is around 8 (alkaline, small intestine enzyme); most enzymes in human cells have an optimum around pH 7.

How does substrate concentration affect rate of reaction? Name the two main types of enzyme-catalysed reactions. What is an anabolic reaction?

Rate increases in direct proportion to substrate concentration until a point (X) is reached, after which increasing substrate concentration further does not increase the rate, because the enzyme’s active sites are saturated (all occupied at any given time). The two main types are anabolic reactions (building up) and catabolic reactions (breaking down). An anabolic reaction combines simpler substances to form more complex molecules and usually requires energy.

What is a catabolic reaction? Give three examples of enzyme inhibitors. What is competitive inhibition?

A catabolic reaction breaks down more complex organic molecules into simpler substances, and energy is usually released. Examples of enzyme inhibitors include cyanide, cadmium, mercury, lead, arsenic, and carbon monoxide (any three). Competitive inhibition occurs when an inhibitor binds directly to the active site of the enzyme, so the substrate is unable to bind to the active site.

What is non-competitive inhibition? Give three commercial/industrial uses of enzymes in the food processing industry. How are enzymes used in the textile industry?

Non-competitive inhibition occurs when an inhibitor binds to a part of the enzyme other than the active site, changing the shape of the enzyme (and its active site) so the substrate is unable to bind. In food processing: protease tenderises meat; lactase catalyses hydrolysis of lactose (for lactose-free milk/ice cream); cellulase breaks down cellulose to remove seed coats from cereal grains and to extract agar from seaweed. In textiles: amylase removes starch used as a stiffener from fabrics, and cellulase softens cotton fabrics in a process called “biopolishing.”

How are enzymes used in detergents? What does the acronym SLAP stand for when drawing graphs? What are the key rules for choosing a Scale?

In detergents, protease and amylase help digest protein and starch stains in clothes, respectively. SLAP stands for Scale, Line, Axis, Points. For Scale: maximize use of the graph paper (at least 50% of the space); find the range (highest value − lowest value); use a sensible ratio (e.g., 1 large square = 10°C); avoid odd scales (e.g., 1 square = 7°C) since they make points hard to read.

What are the rules for the Line on a graph? What are the rules for Axis? What are the rules for plotting Points?

For Line: draw a smooth line/curve of best fit (with data points roughly equally above and below), and only draw the line across the range for which you have data (don’t extrapolate). For Axis: place the independent variable on the x-axis and the dependent variable on the y-axis, and indicate units on each axis (e.g., “Height of plant / cm”). For Points: mark points clearly with a cross (×); use 5 or more points for a good graph; if a point lies far from the best-fit line/curve, it’s an anomalous point and may be ignored.

In a well-plotted graph, why is labelling two pairs of coordinates on the best-fit line useful? Does a graph’s axis have to start at 0?

Labelling two pairs of coordinates on the best-fit line helps you calculate the gradient of the straight line. A graph’s axis does not have to start at 0 — the starting value of each axis is always labelled, but the graph need not start at 0.