Question

Which is correct for bisubstrate reactions?

• A. They are classified into sequential and double displacement reactions
• B. The most widely used rate expression for them is Michaelis- Menten kinetics
• C. Enzyme concentration is in excess of the substrate in the medium
• D. The rate is mainly limited by the substrate concentration

Hint:

• Bisubstrate reactions involve two substrates.
• Sequential mechanism Both substrates bind before product release (can be ordered or random). Ternary complex (EAB) is formed.
• Ping-Pong (Double Displacement) mechanism First substrate binds, product is released, enzyme is modified. Second substrate binds, second product is released, enzyme regenerated. No ternary complex.
• Michaelis-Menten equation is primarily for single-substrate reactions.

Answer 1

The Correct Option is A

Bisubstrate reactions are enzyme-catalyzed reactions involving two substrates and typically yielding two products (e.g., A + B \(\rightleftharpoons\) P + Q). (a) "They are classified into sequential and double displacement reactions": TRUE.

• Sequential Reactions: All substrates must bind to the enzyme before any product is released. This can be ordered (substrates bind in a specific sequence) or random (substrates can bind in any order).
• Double Displacement (Ping-Pong) Reactions: One or more products are released before all substrates have bound. The enzyme is temporarily modified (e.g., by carrying a group from the first substrate to the second).

(b) "The most widely used rate expression for them is Michaelis-Menten kinetics": Michaelis-Menten kinetics was originally derived for single-substrate enzyme reactions. While the principles can be extended to bisubstrate reactions under certain simplifying assumptions (e.g., one substrate is saturating), the rate expressions for bisubstrate reactions are generally more complex (e.g., involving multiple K\(_m\) and V\(_{max}\) terms, or using Cleland notation for rate equations). So, this is generally FALSE as the "most widely used" for *bisubstrate* specifically. (c) "Enzyme concentration is in excess of the substrate in the medium": Enzyme kinetic studies, including Michaelis-Menten, are typically performed under conditions where substrate concentration [S] is much greater than enzyme concentration [E] (\([S] \gg [E]\)), so that the formation of enzyme-substrate complex is dependent on [S]. This statement is FALSE. (d) "The rate is mainly limited by the substrate concentration": This is true at low substrate concentrations for Michaelis-Menten kinetics (first-order with respect to [S]). At high substrate concentrations, the rate becomes limited by the enzyme concentration (zero-order with respect to [S]). For bisubstrate reactions, the rate depends on the concentrations of both substrates in a more complex way. This statement is too general and not always correct, especially not as a defining characteristic. Therefore, the most accurate and defining correct statement for bisubstrate reactions among the options is (a). \[ \boxed{\text{They are classified into sequential and double displacement reactions}} \]