Enzymes & Metabolism (DP IB Biology)

The active is labeled as follows:

Induced-fit binding is where both substrate and enzymes change shape when binding occurs.

Movement is needed for a substrate molecule and an active site to come together. Movement is the result of the kinetic energy that the molecules have which allows an enzyme to collide with a substrate.

During denaturation, the bonds (e.g. hydrogen bonds) holding the enzyme molecule in its precise 3D shape start to break. This means the 3D shape of the enzyme changes. Therefore the shape of the active site is permanently changed, preventing the substrate from binding

The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s). The active site of an enzyme has a specific shape to fit a specific substrate.

The active site is a precise 3D shape which ensures enzyme-substrate specificity. If an enzyme is denatured (due to high heat or extreme pH) then this specific shape is changed and a substrate can no longer bind with the enzyme.

A substrate is a molecule that binds with an enzyme with the enzyme's active site. Substrates bind to enzymes, forming a temporary enzyme-substrate complex. Some enzymes have two substrates that must each collide with a separate active site at the same time

At low temperatures, enzymes work slowly due to a lack of kinetic energy and collisions between the substrates and active sites.

Increasing the temperature towards the optimum increases enzyme activity as the molecules have more kinetic energy, leading to more collisions with substrate molecules and a faster rate of reaction.

Heating enzymes beyond the optimum temperature will break the bonds that hold the enzyme together, causing it to lose its shape and become denatured.

The blue lines show the optimum temperature (approximately 34 ^oC). The rate of reaction is at its maximum at the optimum temperature. After this point, the enzyme begins to become denatured and the rate of reaction decreases.

The rate of an enzyme-controlled reaction decreases as the pH level moves further from the enzyme's optimum because extremes of pH break the bonds that hold the amino acid chain together, denaturing the active site and reducing the rate of reaction.

False.

Different enzymes function well at different pH levels, e.g. enzymes from the stomach have a low optimum pH (around pH 2) because the stomach is an acidic environment.

False.

Some enzymes that are produced in acidic conditions have a lower optimum pH, while those produced in alkaline conditions have a higher optimum pH.

The graph shows an increase in enzyme activity rate as pH increases, up to pH 8.5. This is the optimum pH. After this pH the enzyme begins to denature.

When the concentration of substrate is increased the rate of enzyme activity increases. This is because there are more substrate molecules to collide with the enzyme and occupy the active site.

At point X on the graph:

• Active sites are saturated with substrates

• The rate of reaction has plateaued/stopped

• Any further increase of substrate concentration will not alter the rate of reaction

True.

Generalised sketches of relationships are examples of models in biology. These models can be evaluated using results from enzyme experiments.

The rate of reaction can be determined by measuring the rate of disappearance of a substrate or the rate of product accumulated in a given time period.

False.

A reaction takes place in a short period of time; therefore it has a high rate of reaction.

A high rate of reaction is when the reaction happens in less time i.e. it is faster.

A low rate of reaction is when the reaction happens in more time i.e. it is slower

A steep line on a graph of time against the product made indicates a high rate of reaction.

The units for rate of reaction where the volume of product is measured in cm³ and time is measured in seconds are:

• cm³ per second (volume per second)

• cm³/sec

• cm³ sec^-1

These units all mean the same thing.

Activation energy is the amount of energy needed by the substrate to become unstable enough for a reaction to occur and for new products to be formed.

Enzymes lower the activation energy required for a reaction to take place.

True.

Energy is required to break bonds within the substrate. Enzymes speed up chemical reactions because they reduce the stability of bonds in the substrate.

There is an energy yield when bonds are made to form the products of an enzyme-catalysed reaction.

The graph is labelled as follows:

An intracellular enzyme-catalysed reaction is a reaction that occurs within cells, facilitated by enzymes.

Glycolysis and the Krebs cycle are examples of intracellular metabolic pathways.

True.

Chemical digestion in the gut is an extracellular enzyme-catalysed reaction.

An extracellular enzyme-catalysed reaction is a reaction that occurs outside cells, often in spaces like the digestive tract.

Heat generation, an inevitable outcome of metabolism, helps mammals, birds, and other animals maintain constant body temperatures.

False.

Metabolic reactions are not 100% efficient, and energy is lost as heat.

A cyclical pathway is a metabolic pathway that regenerates its starting molecule, as seen in the Krebs cycle.

Glycolysis is an example of a linear metabolic pathway.

The Krebs cycle is a cyclical metabolic pathway that produces energy in the form of ATP, NADH, and FADH₂.

True.

The Calvin cycle is a cyclical pathway used in photosynthesis to convert CO₂ into glucose.

An allosteric site is a specific site on an enzyme (other than the active site) where only specific substances can bind, causing a conformational change that affects the active site.

False.

Binding to an allosteric site is reversible and causes temporary conformational changes.

True.

Statins are competitive inhibitors that bind reversibly to the active site of enzymes involved in cholesterol synthesis.

Non-competitive inhibition is when an inhibitor binds to an allosteric site of the enzyme required in cholesterol synthesis. This causes changes that prevent catalysis at the active site and therefore reduce cholesterol.

Competitive inhibition is a form of inhibition where an inhibitor (with a similar shape to the substrate) binds reversibly to an enzyme’s active site, blocking the substrate.

Increasing substrate concentration can reduce the effect of competitive inhibition, as more substrate competes with the inhibitor for access to the active site.

Feedback inhibition is a regulatory mechanism where an end product of a metabolic reaction acts as a non-competitive inhibitor of an enzyme to prevent further production.

True.

Isoleucine is an amino acid used by cells in protein synthesis. Its production is regulated by feedback inhibition, where isoleucine itself inhibits its own synthesis pathway.

Mechanism-based inhibition occurs when an inhibitor binds irreversibly to the active site through covalent bonding. This causes chemical changes and the formation of a stable inhibitor-enzyme complex which acts to permanently inactivate the enzyme.

Penicillin irreversibly binds to bacterial transpeptidases which are required in the formation of cross-links between peptidoglycan layers. Without these cross-links, cell wall synthesis is inhibited in bacteria.