The maximum theoretical yield of lysine on glucose. In Problem 4.2it was shown that a growth yield..

The maximum theoretical yield of lysine on glucose. In
Problem 4.2it was shown that a growth yield of 6/7 for L-lysine on glucose
(Example 3.9) could not be excluded for thermodynamic reasons. In the present
problem it will be shown that the explanation for the lower theoretical yield
(3/4) – see references to Problem4.2 – is to be found in the need for NADPH and
not NADH in the synthesis of lysine from glucose. This will help you to appreciate
that the crude definition of redox equivalents as “H2” cannot always
be used.

(a) Go through the biosynthesis pathway inC. glutamicum of
L-lysine

The maximum theoretical yield of lysine on glucose. In
Problem 4.2it was shown that a growth yield of 6/7 for L-lysine on glucose
(Example 3.9) could not be excluded for thermodynamic reasons. In the present
problem it will be shown that the explanation for the lower theoretical yield
(3/4) – see references to Problem4.2 – is to be found in the need for NADPH and
not NADH in the synthesis of lysine from glucose. This will help you to appreciate
that the crude definition of redox equivalents as “H2” cannot always
be used.

(a) Go through the biosynthesis pathway inC. glutamicum of
L-lysine from 1 mol of pyruvate and 1 mol of phosphoenol pyruvate. For each
step write down the chemical formula for the intermediate and the amount of
redox units harbored in the compound. Show that the net consumption rate of
CO2and of

2-oxoglutarate is zero, and that four NADPH is consumed
while two NADH is produced for each mole of L-lysine produced from glucose.
Prove that this leads to a theoretical yield of 3/4 C-mole lysine

(C-mol glucose)1, and that 1/6 O2must be used for each
C-mole lysine produced.

(b) Can any of the NADPH requiring enzymes in the pathway
from glucose tolysine possibly be exchanged with an enzyme that uses NADH as
cofactor? Or could an NADH producing enzyme be exchanged with an NADPH
producing iso enzyme? What would be the possible thermodynamic consequences of exchanging
the glutamate dehydrogenase encoded by GDH1 (using NADP Has cofactor) with the
GDH2 encoded iso enzyme (using NADH to produce glutamic acid from
2-oxoglutarate)?

To solve this problem use the following references:

1. “Biochemical Pathways” ed. G. Michal, Spektrum Verlag,
Berlin, 19992. “Ex PASy-Molecular Biology Server, Subsection Biological
Pathways” This reference is found at:http://www.expasy.ch.