Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 396–404 of the course textbook. Image credits are on slide 14.
Most enzymes work inside the cell that made them, for example in glycolysis (cytoplasm) and the Krebs cycle (mitochondrial matrix). Some are secreted by exocytosis and work outside the cell, such as the digestive enzymes that break down food in the gut.
No energy transfer in metabolism is 100% efficient, so heat is always released. Mammals and birds use this heat to keep a constant body temperature, and can raise it by shivering or by brown fat, which releases energy from respiration as heat.
Metabolic pathways are series of enzyme-catalysed steps. In a linear pathway, such as glycolysis, a substrate is converted to an end product. In a cyclical pathway, such as the Krebs cycle and the Calvin cycle, the last step regenerates the first compound, so a small amount of it can process many molecules.
An allosteric site is a binding site away from the active site. Only specific molecules fit it. Binding changes the shape of the enzyme so the active site can no longer catalyse the reaction. Binding is reversible, and adding more substrate does not overcome the inhibition, so the maximum rate is lowered.
A competitive inhibitor resembles the substrate and binds reversibly to the active site. At high substrate concentration the substrate outcompetes it, so the maximum rate can still be reached. Statins inhibit HMG-CoA reductase in this way, reducing cholesterol synthesis in the liver and lowering LDL in the blood.
In bacteria and plants, isoleucine is made from threonine in a pathway of several steps. When isoleucine builds up, it binds to the allosteric site of the first enzyme, threonine deaminase, and slows the whole pathway. As isoleucine is used, it leaves the enzyme and production restarts. This negative feedback avoids wasting resources.
Bacteria use transpeptidases to cross-link peptidoglycan in their walls. Penicillin enters the active site, its four-membered ring opens and it forms a covalent bond with the enzyme, permanently inactivating it. Bacteria with a changed transpeptidase, whose active site binds penicillin poorly but still cross-links the wall, are resistant.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all seven statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Toothpicks as substrate, hands as enzymes, paper clips as inhibitor, timer
Safety: Eye protection, since toothpick pieces can fly. Keep hands away from broken ends.
Carry this out with your teacher's supervision and follow your school's lab rules.
In 1913 Leonor Michaelis and Maud Menten described enzyme kinetics with an equation that is still in use.
The model assumes conditions rarely found in cells. Consider how something known to be simplified can still be reliable.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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