Lactate dehydrogenase, aldehyde dehydrogenase, and glutamate dehydrogenase are important metabolic enzymes. Lactate dehydrogenase catalyzes the interconversion of lactate and pyruvate via NADH. It exists in different isoforms that localize to specific tissues. Aldehyde dehydrogenase oxidizes aldehydes to carboxylic acids using NAD(P)+. It is found in the liver and other tissues and plays a role in ethanol metabolism. Glutamate dehydrogenase converts glutamate and α-ketoglutarate using NAD(P)H and is important for amino acid metabolism, urea synthesis, and insulin secretion regulation. All three enzymes play key roles in cellular metabolism and their levels/activities can indicate metabolic states

1. Structure and Function of Dehydrogenases

2. Lactate dehydrogenase
In animals, L-lactate is constantly produced from pyruvate in a process of fermentation during
normal metabolism and exercise.
The concentration of lactate is ~1-2 mM at rest, but can rise to over 20 mM during intense
exertion.

3. This occurs due to metabolism in red blood cells that lack mitochondria.
Why is there any lactate at rest?
Limitations resulting from the enzyme activity that occurs in muscle fibers having a high
glycolytic capacity.

4. Enzyme isoforms
Functional lactate dehydrogenase are homo or hetero tetramers composed of M and H protein
subunits encoded by the LDHA and LDHB genes (~75% identity), respectively.
LDH-1 (4H) - heart and RBC
LDH-2 (3H1M) - reticuloendothelial system
LDH-3 (2H2M) - lungs
LDH-4 (1H3M) - kidneys, placenta, pancreas
LDH-5 (4M) – liver, striated muscle

5. S-lactate + NAD+ ↔ pyruvate + NADH
1. NAD-dependent
1. EC: 1.1.1.27 – acting on L-lactate (S-lactate)
2. EC: 1.1.1.28 – acting on D-lactate (R-lactate)
Four classes of lactate dehydrogenase

6. 2. Cytochrome c-dependent
1. EC: 1.1.2.3 – acting on L-lactate (S-lactate)
2. EC: 1.1.2.4 – acting on D-lactate (R-lactate)
S-lactate + 2 ferricytochrome c ↔ pyruvate + 2 ferrocytochrome c + 2 H+.

7. Why is there a requirement of four enzymes performing the same function?

8. Cytochrome c-dependent LDH
PDB id: 1KBI

9. PDB id: 1I10
NAD-dependent LDH

10. In anaerobic cells, the ratio of pyruvate/lactate is much less than 1 while under aerobic
conditions the ratio of pyruvate/lactate is much greater than 1. Why?
In the absence of oxygen (anaerobic), the conversion of pyruvate to lactate is the only reaction
that can regenerate NAD+ allowing further glycolysis.

11. Cytochrome c-dependent LDH contains two domains: (1) Cytochrome binding and (2) FMN
binding.
The enzyme is a soluble component of the mitochondrial intermembrane space, where it
catalyses the reduction of pyruvate to lactate.
The enzyme transfers electrons resulting from
the oxidation of lactate into pyruvate directly
to cytochrome c.

13. L- Vs D- NAD-dependent LDH
PDB id: 3KB6PDB id: 1I10

14. Lactic acid biosynthesis

15. Reduction of pyruvate by L-LDH

16. Reduction of pyruvate by D-LDH

17. Mechanism of enzymatic action

18. Disease associated with lactic acid production
Lactic acid producing bacteria can grow in the mouth responsible for the tooth decay known
as caries.
Glucose vs Lactic acid
In brain metabolism, lactate is proposed to be the main source of energy metabolized by
neurons in the brain of several mammals species including humans.

19. Aldehyde dehydrogenase
EC: 1.2.1.3 An aldehyde + NAD+ + H2O = a carboxylate + NADH
EC: 1.2.1.4 An aldehyde + NADP+ + H2O = a carboxylate + NADPH

20. Locations and function
Aldehyde dehydrogenase is a polymorphic enzyme mostly found in the liver. Carboxylic acid
produced in the liver are metabolized by the body’s muscle and heart.
In addition, these enzymes are found in many other tissues of the body.
There are three different classes of these enzymes in mammals: class 1 (low Km, cytosolic),
class 2 (low Km, mitochondrial) and class 3 (high Km, such as those expressed in tumors,
stomach and cornea).

21. To date, nineteen ALDH genes have been identified within the human genome.
Biological unit

22. NAD binding site
Cys302 and Glu268 interact with the
aldehyde substrate.

23. Mechanism of action
RCHO + NAD+ + H2O → RCOOH + NADH + H+

24. Glutamate dehydrogenase
EC: 1.4.1.2 L-glutamate + H2O + NAD+ = 2-oxoglutarate + NH3 + NADH
EC: 1.4.1.4 L-glutamate + H2O + NADP+ = 2-oxoglutarate + NH3 + NADPH

25. GDH converts glutamate to α-ketoglutarate and vice versa and are required for urea synthesis.
These are present in most microbes and the mitochondria of eukaryotes. In animals, the
produced ammonia is usually used as a substrate in the urea cycle.
Location and function

26. Structure of GDH

27. Reaction catalyzed by GDH
Deamination of amino acids to the appropriate ketone

28. Mechanism of enzymatic action

29. Regulation of GDH
Under low blood glucose and caloric restriction, GDH activity is raised in order to increase
the amount of α-ketoglutarate which can be used in the citric acid cycle to ultimately produce
ATP.
In humans, the activity of GDH in insulin-producing β cells is monitored through ADP-
ribosylation, a covalent modification carried out by the gene SIRT4.
β cells secrete insulin in response to an increase in the ATP:ADP ratio. As amino acids are
broken down by GDH into α-ketoglutarate, this ratio rises and more insulin is secreted. SIRT4
is necessary to regulate the metabolism of amino acids as a method of controlling insulin
secretion and regulating blood glucose levels.
In microbes, the activity of GDH is controlled allosterically by the binding of ammonium
and/or rubidium ion.

30. Clinical application
GDH is important for distinguishing between acute viral hepatitis and acute toxic liver
necrosis or acute hypoxic liver disease, particularly in the case of liver damage with very high
aminotransferases.
In clinical trials, GDH can serve as a measurement for the safety of a drug.
Elevated blood serum GDH levels indicate liver damage. Thus, GDH can play a role in the
differential diagnosis of liver disease.