Harvesting the Outcome of Glycolysis: Controlling the Fate of Pyruvate

Controlling the Fate of Pyruvate

Overview

The fate of pyruvate, the end product of glycolysis, is tightly controlled. It can either be oxidized to Acetyl CoA, leading into the citric acid cycle or fatty acid synthesis, or it can be retained for gluconeogenesis, ultimately forming glucose or glycogen. This control is achieved through various enzymes, isoenzymes, and their tissue-specific expression.

Pyruvate Dehydrogenase (PDH) and Pyruvate Carboxylase (PC)

Key enzymes that determine the fate of pyruvate include:

  • Pyruvate Dehydrogenase (PDH): Converts pyruvate to Acetyl CoA.
  • Pyruvate Carboxylase (PC): Converts pyruvate to oxaloacetate, a precursor for gluconeogenesis and replenishing the citric acid cycle.
  • PEP Carboxykinase (PEPCK): Converts oxaloacetate to PEP (phosphoenolpyruvate), a key step in gluconeogenesis.
  • Pyruvate Kinase (PK): Converts PEP to pyruvate in glycolysis.

Mitochondrial Fate of Pyruvate

In the mitochondria, pyruvate can undergo two primary fates:

  1. Conversion to Acetyl CoA: Via pyruvate dehydrogenase. This Acetyl CoA can then enter the citric acid cycle for oxidation or be used for the synthesis of fatty acids, sterols, triglycerides, and phospholipids.
    Pyruvate+CoA+NAD+→Acetyl CoA+CO2+NADHPyruvate + CoA + NAD^+ \rightarrow Acetyl \ CoA + CO_2 + NADH

  2. Conversion to Oxaloacetate: Via pyruvate carboxylase. Oxaloacetate can replenish citric acid cycle intermediates or be used for glucose synthesis via gluconeogenesis.

This process occurs in the liver, kidney, muscle, and adipose tissues, and in all cells with mitochondria.

Regulation of Pyruvate Dehydrogenase (PDH)

PDH is a multienzyme complex whose activity is regulated through phosphorylation. Phosphorylation inactivates PDH, while dephosphorylation activates it.

  • PDH Kinase: Phosphorylates and inactivates PDH.
  • PDH Phosphatase: Dephosphorylates and activates PDH.
Allosteric Regulation of PDH Kinase

PDH kinase is allosterically regulated by various metabolites:

  • Activators: Acetyl CoA and NADH (products of PDH) activate PDH kinase, leading to feedback inhibition.
  • Inhibitors: Pyruvate, CoA, and NAD+ (substrates of PDH) inhibit PDH kinase, favoring continued PDH activity. ADP, an indicator of low cellular energy status, also inhibits PDH kinase.
Hormonal Regulation of PDH Phosphatase

PDH phosphatase is activated by insulin signaling, indicating an influx of glucose-derived pyruvate. This coordinates glucose utilization with fatty acid metabolism, particularly during starvation.

Glucose-Fatty Acid Cycle

Fatty acid oxidation inhibits glycolysis at multiple points:

  • Inhibition of phosphofructokinase (PFK) by citrate.
  • Inhibition of pyruvate dehydrogenase (PDH) by increased Acetyl CoA levels.

Citric Acid Cycle and Regulation

The citric acid cycle is regulated at several key steps, including:

  • Citrate Synthase: Condensation of oxaloacetate and acetyl CoA to form citrate. ATP inhibits it.
  • α-ketoglutarate Dehydrogenase: Converts α-ketoglutarate to succinyl CoA, and is inhibited by ATP, succinyl CoA and NADH.

These regulatory mechanisms coordinate the oxidation of glucose, fatty acids, ketones, and amino acids.

Pyruvate Carboxylase (PC) Regulation

PC is allosterically activated by acetyl CoA, which increases during fatty acid oxidation in starvation. PC gene expression is also regulated:

  • Increased by glucagon (via protein kinase A) and glucocorticoids (via nuclear receptors).
  • Decreased by insulin signaling.

These hormonal changes enhance the liver's capacity for gluconeogenesis during starvation.

Phosphoenolpyruvate (PEP)/Pyruvate Cycle

The PEP/pyruvate cycle involves the interconversion of PEP and pyruvate and is critical for regulating gluconeogenesis and glycolysis. Key enzymes involved:

  • PEP Carboxykinase (PEPCK): Converts oxaloacetate to PEP in gluconeogenesis.
  • Pyruvate Kinase (PK): Converts PEP to pyruvate in glycolysis.
  • Pyruvate Carboxylase (PC): Converts pyruvate to oxaloacetate, thus feeding PEP production in gluconeogenesis
  • Pyruvate Dehydrogenase (PDH): Converts pyruvate to Acetyl CoA.
Fates of Pyruvate
  • Loss to Oxidation: Pyruvate can be converted to Acetyl CoA and enter the citric acid cycle or be used for fatty acid synthesis.
  • Retention via Gluconeogenesis: Pyruvate can be converted to glucose or glycogen.
PEPCK Isoenzymes

PEPCK exists in two isoforms:

  • Cytosolic: Involved in converting amino acids to glucose.
  • Mitochondrial: Involved in lactate recycling (Cori cycle).

These isoforms are encoded by separate genes and exhibit tissue-specific expression in the liver, kidney, skeletal muscle, intestinal mucosa, and adipose tissue.

Cori Cycle and Glucose-Alanine Cycle
  • Cori Cycle: Lactate produced in muscle is transported to the liver, where it is converted to glucose via gluconeogenesis.
  • Glucose-Alanine Cycle: Alanine produced in muscle is transported to the liver and converted to glucose via gluconeogenesis.

Regulation of PEPCK Gene Expression

The PEPCK gene promoter is regulated by several transcription factors:

  • Positive Regulators: Glucagon (via CREB and protein kinase A) and glucocorticoids (via nuclear receptors) enhance transcription.
  • Negative Regulators: Insulin (via SREBP-1c) decreases transcription.

This regulation enhances liver gluconeogenesis during starvation by converting amino acids released from muscle proteolysis into glucose.

Metabolic State Control

  • Fed State: Insulin promotes glycolysis and glycogen synthesis.
  • Starved State: Glucagon promotes gluconeogenesis and glycogen breakdown.