Lactate dehydrogenase

Question 1

What are the two important residues in the LDH active site and what is their role?

Answer 1

Arg171 = coordinates the carboxylic acid group of pyruvate

His195 directly contributes to catalysis by aiding in the wave of electrons from NADH nicotinamide ring to the His imidazole ring and subsequent reduction of pyruvate to lactate.

Question 2

What are the two differences between LD1 (heart) and LD5 (liver and muscle)?

Answer 2

LD1 has low Km (high affinity) and is allosterically inhibited by pyruvate as feedback inhibition

LD5 has a high Km and isn’t subject to feedback inhibition

Question 3

Why does the heart primarily have LD1 isoform?

Answer 3

The heart is a highly aerobic organ with a high
energy demand and plasma membrane permeable to lactate and pyruvate.

• high affinity of LD1 for lactate means it is effectively scavenged and converted to pyruvate, which can then be oxidised to acetyl CoA and go through CA cycle to produce ATP
• the reverse reaction is allosterically inhibited by pyruvate

Question 4

Why does the muscle primarily use LD5 isoform?

Answer 4

during intensive exercise, ATP is primarily produced by anaerobic glycolysis to produce pyruvate.
LD5 converts reduces pyruvate to lactate to regenerate the NAD+ cofactor required for glycolysis.
- LD5 has low affinity as [pyruvate] is always high here

Question 5

Why does the liver primarily use LD5 isoform?

Answer 5

lactate from muscles elevates in blood and is taken up by the hepatocytes where it is converted back to pyruvate to channelled into gluconeogenesis
-LD5 has low affinity = only want to make glucose when [lactate] is high enough.

Question 6

Liver and ______ = _____ direction, ________ isoform

Liver and ______ = _______ direction, _______ isoform

Answer 6

Liver and heart = same direction (lactate to pyruvate), different isoform (heart LD1, liver LD5)

Liver and muscle = different direction (liver lactate to pyruvate, muscle pyruvate to lactate = Cori cycle), same isoform (LD5)