Factors and inhibition

MDCAT Biology · Biology · Enzymes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All MDCAT Biology topics

The idea

The rate of an enzyme reaction depends on how often substrate molecules meet free active sites, and on whether the active site keeps its shape.

Warming gives molecules more kinetic energy, so the rate rises up to the optimum temperature (about 37 °C for human enzymes). Above it the enzyme is denatured: bonds holding its shape break and the damage is usually permanent. Cold only slows the enzyme; it works again when warmed. Each enzyme also has an optimum pH, because pH changes the charges on its amino acids.

An inhibitor lowers the rate. You must tell a competitive inhibitor from a non-competitive one.

Rules to remember

Enzyme concentration: with excess substrate, rate is directly proportional to the amount of enzyme.
Substrate concentration: rate rises, then levels off at a maximum when every active site is occupied (saturation).
Optimum pH: pepsin about 2, salivary amylase about 6.8, pancreatic lipase about 9.
Competitive inhibitor: resembles the substrate and binds at the active site; more substrate overcomes it, so the maximum rate is still reached.
Non-competitive inhibitor: binds at another site and changes the shape of the active site; more substrate does not help, so the maximum rate is lower.
Irreversible inhibitors (e.g. cyanide) bind by strong covalent bonds; the enzyme stays inactive.
Feedback inhibition: the end product of a pathway slows an early enzyme of that pathway.

Common mistake

Students write that cold denatures enzymes. Low temperature only makes them inactive and this is reversible; high temperature denatures them and this is not.

Worked example

The rate of a human enzyme reaction roughly doubles for each 10 °C rise up to the optimum. The rate is 6 units at 17 °C. Estimate the rate at 37 °C, and say what happens at 70 °C.

17 °C to 27 °C is one 10 °C rise: rate = 6 × 2 = 12 units.
27 °C to 37 °C is a second rise: rate = 12 × 2 = 24 units.
70 °C is far above the optimum of 37 °C, so the active site loses its shape.

Answer: About 24 units at 37 °C. At 70 °C the enzyme is denatured and the rate falls to almost zero.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Enzyme action Theories of evolution →Take a MDCAT Biology test