What are the precursors of gluconeogenesis?

Gluconeogenesis is the formation of glucose from a non-glucose precursor including pyruvate, lactate, and amino acids.

Pyruvate and Lactate:
Pyruvate is the end product of glycolysis which can be further metabolized to either acetyl CoA or lactate. The conversion of pyruvate to these products depends on tissue type, ATP status, and regulatory effector molecules. The conversion of pyruvate to acetyl CoA is an irreversible step that is catalyzed by the enzyme pyruvate dehydrogenase that occurs mainly in oxidative tissues such as the liver, cardiac muscle. 
In contrast, the conversion of pyruvate to lactate is a reversible step catalyzed by lactate dehydrogenase that occurs mainly in non-oxidative tissues such as red blood cells. The lactate dehydrogenase is also present in skeletal muscle that converts pyruvate to lactate during vigorous exercise.


Figure 1: Fates of Pyruvate PDH- Pyruvate dehydrogenase, LDH- Lactate dehydrogenase

Pyruvate and Lactate formed in peripheral tissues shuttled into the liver for gluconeogenesis:
The lactate formed during anaerobic glycolysis can directly be shuttled into the liver. In the liver, the lactate dehydrogenase isoenzyme converts lactate and pyruvate. 
Alternately, the lactate formed during anaerobic glycolysis is reconverted into pyruvate and subsequently converted to alanine by the enzyme alanine transaminase. The lactate or alanine diffuse into the bloodstream and is taken up by the liver. In the liver, alanine is converted by liver alanine transaminase to convert into pyruvate.


Figure 2: Glucose-alanine cycle




Figure 3: Cori cycle in Erythrocytes

Amino acid
Glucogenic amino acids are a group of amino acids whose metabolic intermediates can enter the gluconeogenic pathway and form glucose. Examples include alanine, aspartate, glutamate, etc.

Propionyl CoA
Propionyl CoA an intermediate of odd-chain fatty acids is also the precursor for the gluconeogenic pathway. Propionyl CoA is converted into methyl malonyl CoA by the enzyme propionyl CoA carboxylase ( a biotin-containing enzyme). The methyl malonyl CoA is converted into succinyl CoA by the enzyme methyl malonyl CoA mutase and the succinyl CoA enters the TCA cycle to form oxaloacetate. The oxaloacetate thus formed enters the gluconeogenic pathway to form glucose.

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