Biology · Biomolecules · NEET
NCERT (Ch 9) lists four things that alter the enzyme's tertiary structure and so change its activity: temperature, pH, change in substrate concentration, and binding of specific chemicals that regulate its activity. For NEET, temperature and pH are the most asked. All of them work by affecting the 3D shape of the protein, which changes the active site.
High temperature (say above 40 degrees C for most human enzymes) denatures the protein — heat breaks the bonds holding the 3D shape, so the active site is lost permanently and activity is destroyed. Low temperature does NOT denature; it only slows molecular motion, so the enzyme is preserved in a temporarily inactive state and becomes active again on warming. This exact line is directly from NCERT and is a common NEET statement question.
Each enzyme shows its highest (maximum) activity at one particular temperature and one particular pH. That value is called the optimum temperature and optimum pH. Activity declines both below and above this optimum. Enzymes generally work only in a narrow range of temperature and pH (NCERT, Figure 9.5).
As substrate concentration increases, the reaction rate (velocity) first rises. But it keeps rising only up to a maximum velocity, Vmax. After Vmax no further rise happens even if you add more substrate, because all active sites are busy — the enzyme is saturated. So substrate concentration raises activity only until saturation.
Yes, but only special enzymes. NCERT notes that enzymes isolated from thermophilic organisms living in hot vents and sulphur springs are stable and keep their catalytic power even up to 80-90 degrees C. This thermal stability is an important quality of such enzymes. For a normal human enzyme, however, above about 40 degrees C means denaturation.
Given below are two statements: Statement I: Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat. Statement II: When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor. In the light of the above statements, choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Temperature and pH are the two most asked. Both change the enzyme's tertiary structure and each enzyme has an optimum temperature and optimum pH at which activity is maximum.
Activity falls sharply. Above about 40 degrees C the protein gets denatured by heat, the active site shape is lost, and the enzyme stops working permanently.
Low temperature does not denature at all; it only makes the enzyme temporarily inactive. On warming back, activity returns. Heat denaturation, in contrast, is essentially not reversible.
Because active sites get fully occupied (enzyme saturation). Once all enzyme molecules are working at top speed, adding more substrate cannot raise the rate beyond Vmax.
It is the pH value at which a given enzyme shows its highest activity. Above or below this pH the activity declines because the charge and 3D shape of the enzyme change.
