Q.65 The active site in the alpha/beta barrel structures is usually located
(A) inside the barrel (B) at the amino side of the strands
(C) at the carboxy side of the strands (D) at any arbitrary site
Active Site Location in Alpha/Beta Barrel Structures
The active site in alpha/beta barrel structures, commonly known as TIM barrels, typically resides at the carboxy-terminal ends of the beta-strands, forming a funnel-shaped pocket for substrate binding.
Correct Answer
Option (C) at the carboxy side of the strands is correct. This positioning arises because the loops connecting the C-terminal ends of the parallel beta-strands to the following alpha-helices create a conserved active site cleft in enzymes like triosephosphate isomerase.
Option Explanations
(A) Inside the Barrel
The barrel core consists of tightly packed hydrophobic residues from beta-strands and alpha-helices, providing structural stability rather than catalytic function. Active sites avoid this interior to remain accessible to solvents and substrates.
(B) At the Amino Side of the Strands
The N-terminal ends of beta-strands connect to preceding alpha-helices, forming the barrel’s opposite face without loop extensions suitable for catalysis. This region faces away from the substrate entry point.
(C) At the Carboxy Side of the Strands
Beta-alpha connecting loops at the C-terminal ends of beta-strands protrude outward, forming the active site in nearly all TIM barrel enzymes. This universal feature enables diverse catalytic activities while maintaining scaffold integrity.
(D) At Any Arbitrary Site
Structural conservation dictates active site placement; variability would disrupt folding or function. Over 10% of known enzymes use this fold with predictable site geometry.
Structural Significance
Alpha/beta barrels feature eight alternating beta-alpha units, with beta-strands forming a central cylinder and helices on the exterior. The C-terminal loop region’s flexibility allows functional divergence across superfamilies.
