Question

2. a. Why do cell extracts require NAD⁺ to convert glucose to pyruvate? What intermediates accumulate when NAD⁺ is absent? Explain. b.Why does the rate at which glucose is converted to pyruvate decrease when citrate is added?

Answer 1

NAD+ = Nicotinamide adenine dinucleotide

Glycolysis( splitting sugars) is the process of releasing energy within sugars. In glycolysis, glucose (a six carbon sugar) is split into two molecules of the three-carbon sugar pyruvate. This multi-step process yields two molecules of ATP (free energy containing molecule), two molecules of pyruvate, and two "high energy" electron carrying molecules of NADH.

Now the question is why is NAD+ required in this process of Glycolysis.

NAD+ serves as an electron acceptor. NAD+ accepts 2 electrons and also acquires a proton (H ⁺ ) and is converted into NADH. NADH is a reduced electron carrier. Reaction can be written as:

NADH then becomes oxidized in the first step of electron transport by mitochondrial complex I. The reaction is as follows:

It is during this oxidation that ATP is produced, by the enzymes of the respiratory chain(Occurs in inner membrane of the mitochondria).

In the absence of NAD+, Glycolysis can't continue. Glyceraldehyde-3-phosphate is the intermediate.

When Citric acid(produced in the krebs cycle) levels increase they slow down glycolysis, because there is more than enough substrate to produce the NAD+ needed for energy. The high levels of citric acid also inhibit phosphofructokinase-1(PFK). In simple terms If citrate builds up, glycolysis slows down because the citric acid cycle is backed up and doesn’t need more fuel.