Question

Which of the following represent fermentation of lactose to lactic acid by homofermentative bacteria?

• A. Lactose + 1 H\(_3\)PO\(_4\) + 1 ADP \(\rightarrow\) 2 Lactic Acid + 2 ATP + 2 H\(_2\)O
• B. Lactose + 4 ADP + 4 H\(_3\)PO\(_4\) \(\rightarrow\) 4 Lactic Acid + 4 ATP + 3 H\(_2\)O
• C. Lactose + 2 ADP + 2 H\(_3\)PO\(_4\) \(\rightarrow\) 2 Lactic Acid + 2 ATP + 3 H\(_2\)O
• D. Lactose + 3 ADP + 3 H\(_3\)PO\(_4\) \(\rightarrow\) 3 Lactic Acid + 3 ATP + 3 H\(_2\)O

Hint:

One mole of lactose contains two hexose units.
Since homofermentative glycolysis yields 2 ATP per hexose unit, the complete fermentation of one mole of lactose must yield 4 ATP and 4 lactic acid molecules.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Homofermentative lactic acid bacteria (such as Lactococcus lactis) ferment sugars exclusively through the Embden-Meyerhof-Parnas (glycolytic) pathway, converting hexoses almost quantitatively into lactic acid with the generation of ATP.

Step 2: Detailed Explanation:
Let us analyze the stoichiometry of lactose fermentation:
Lactose is a disaccharide consisting of glucose and galactose.
In homofermentative dairy lactococci, lactose is internalized via a phosphoenolpyruvate-dependent phosphotransferase system (PEP-PTS) as lactose-6-phosphate, which is then cleaved into glucose and galactose-6-phosphate.
- The glucose molecule enters glycolysis, yielding \( 2\text{ moles} \) of lactic acid and a net of \( 2\text{ moles} \) of ATP.
- The galactose-6-phosphate molecule is metabolized via the tagatose-6-phosphate pathway, which also yields \( 2\text{ moles} \) of lactic acid and a net of \( 2\text{ moles} \) of ATP.
Combining both pathways, the fermentation of \( 1\text{ mole} \) of lactose yields exactly:
- \( 4\text{ moles} \) of lactic acid (lactate),
- \( 4\text{ moles} \) of ATP (from \( 4\text{ ADP} \) and \( 4\text{ H}_3\text{PO}_4 \)).
This metabolic equation is represented by:
\[ \text{Lactose} + 4\text{ ADP} + 4\text{ H}_3\text{PO}_4 \rightarrow 4\text{ Lactic Acid} + 4\text{ ATP} + 3\text{ H}_2\text{O} \]
This matches option 2.

Step 3: Final Answer:
The correct equation is option 2.