Question

Chlorophyll biosynthesis begins with this amino acid

• A. Glycine
• B. Glutamic acid
• C. Aspartic acid
• D. Alanine

Hint:

Do not confuse animal heme synthesis with plant chlorophyll synthesis:
- Animal/Fungal pathway $\rightarrow$ Starts with Glycine and Succinyl-CoA.
- Plant/Algal pathway $\rightarrow$ Starts with Glutamic acid ($\text{C}_5$ pathway).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Chlorophyll is the primary pigment responsible for light absorption during photosynthesis.
Structurally, it is a tetrapyrrole containing a central magnesium atom.
The biochemical pathway of chlorophyll biosynthesis in plants begins with the synthesis of delta-aminolevulinic acid (ALA), which is the universal precursor for all tetrapyrroles.

Step 2: Detailed Explanation:
Let us examine the two biological pathways for ALA synthesis:
In animals, fungi, and some bacteria, ALA is synthesized via the Shemin pathway (condensation of glycine and succinyl-CoA catalyzed by ALA synthase), which matches Option (A).
However, in plants, algae, and most bacteria, chlorophyll and heme biosynthesis utilizes a completely different pathway known as the $\text{C}_5$ pathway (or Beale pathway).
This pathway begins with the amino acid Glutamic acid (Glutamate).
The $\text{C}_5$ pathway involves three enzymatic steps:
First, Glutamate is activated by coupling to a transfer RNA ($tRNA^{\text{Glu}}$) by glutamyl-tRNA synthetase.
Second, glutamyl-tRNA reductase reduces the glutamyl-tRNA to glutamate-1-semialdehyde.
Third, glutamate-1-semialdehyde aminotransferase rearranges this intermediate to form delta-aminolevulinic acid (ALA).
Subsequent condensation of eight ALA molecules forms the protoporphyrin ring, which is then chelated with magnesium to yield chlorophyll.
Therefore, in higher plants, chlorophyll biosynthesis begins with Glutamic acid.

Step 3: Final Answer:
Chlorophyll biosynthesis in plants begins with Glutamic acid.
This corresponds to Option (B).