topic 9

Energy metabolism is a continuous process within the body, essential for generating energy and sustaining life functions. Key metabolic intermediates that frequently appear across various pathways include glucose-6-phosphate, pyruvate, and acetyl-CoA. The overarching goal of these processes is often the production of adenosine triphosphate (ATP).

The Three Sisters: Nutritional Complementarity

Many Indigenous peoples in North America traditionally cultivate corn, beans, and squash together. This practice, highlighted in Braiding Sweetgrass by Dr. Robin Wall Kimmerer, leverages the ecological and nutritional synergy of these crops:

Corn: Primarily provides starch, converting sunlight into carbohydrates for food energy. It is not nutritionally complete on its own.
Beans: Rich in protein due to their nitrogen-fixing capabilities, they supplement the nutritional deficiencies of corn.
Squash: Offers essential vitamins through its carotene-rich flesh, which can be converted into retinol (Vitamin A), crucial for eyesight.

Together, these plants form a comprehensive nutritional triad. The niacin (Vitamin B3) in corn, which is typically not readily absorbed, becomes bioavailable through nixtamalization. This traditional Indigenous chemical process involves:

Treating corn with a highly alkaline solution (e.g., from wood ash and water).
Chemically dissolving the corn's hull.
Converting bound niacin into free, absorbable niacin.
Introducing absorbable calcium from the wood ash.

Topic 9: Carbohydrate Metabolism

Carbohydrate metabolism encompasses several interconnected pathways focused on glucose utilization and storage.

1. Glycogen Metabolism

Glycogen, a polysaccharide composed of numerous glucose residues, serves as an energy storage molecule. Both its synthesis and degradation are rigorously regulated.

Glycogen Synthesis (Glycogenesis)

Starting point: Glucose-6-phosphate (derived from ingested and phosphorylated glucose).
First step: A phosphate group is moved from carbon 6 to carbon 1, forming glucose-1-phosphate. This is an equilibrium reaction.
Polymerization: Glucose-1-phosphate is then added to a growing glycogen chain, catalyzed by glycogen synthase.
Energy requirement: This step utilizes UTP hydrolysis to UDP and inorganic phosphate (Pi), making the reaction energetically favorable ( $ext{negative} riangle ext{G}$ ). UTP functions similarly to ATP as an energy donor.

Glycogen Degradation (Glycogenolysis)

Enzyme: Glycogen phosphorylase breaks down glycogen.
Mechanism: Inorganic phosphate (Pi) is used to cleave individual glucose units from glycogen, yielding glucose-1-phosphate.
Energy: This reaction is inherently favorable ( $ext{negative} riangle ext{G}$ ) and does not require UTP hydrolysis.
Distinct Pathways: Glycogen synthesis and degradation involve different enzymes and distinct reaction mechanisms. This allows both processes to proceed with a net $ext{negative} riangle ext{G}$ in their respective directions, preventing a futile cycle where energy would be wasted.

1.1. Regulation of Glycogen Metabolism

Regulation primarily targets the irreversible steps catalyzed by glycogen synthase and glycogen phosphorylase, particularly those involved in $ext{alpha-1,4}$ linkages.

Allosteric Regulation

Glucose-6-phosphate: In skeletal muscle, it acts as an allosteric activator of glycogen synthase and an allosteric inhibitor of glycogen phosphorylase. High levels signal abundant glucose, promoting storage and inhibiting breakdown.
ATP & AMP (Skeletal Muscle):
- High ATP: Inhibits glycogen phosphorylase. Sufficient energy means no immediate need to break down glycogen.
- High AMP: Activates glycogen phosphorylase. High AMP indicates low ATP (energy deficit), prompting glycogen breakdown for ATP production.
ATP & AMP (Liver): Liver glycogen phosphorylase is not regulated by ATP or AMP. The liver's role for glycogen is to maintain blood glucose for the body (especially the brain), not for its own direct energy needs.
Glucose (Liver Specific): Glucose, rather than glucose-6-phosphate, is an allosteric inhibitor of liver glycogen phosphorylase. This directly reflects blood glucose levels; high glucose signals no need for glycogen breakdown.

Hormonal Regulation (Covalent Modification - Phosphorylation/Dephosphorylation)

Insulin (High Blood Glucose):
- Released by the pancreas in response to high blood glucose.
- Triggers a signal cascade (indirectly, via receptor binding) that leads to dephosphorylation of key enzymes.
- Activates glycogen synthase (promotes glucose storage).
- Inactivates glycogen phosphorylase (inhibits glycogen breakdown).
- Effect in both liver and muscle: Both tissues respond to insulin by storing glucose as glycogen. This does not preclude cells from simultaneously utilizing glucose for immediate energy needs.
Glucagon (Low Blood Glucose):
- Released when blood glucose levels fall, signaling a need to raise glucose.
- Triggers a signal cascade that leads to phosphorylation of key enzymes.
- Inactivates glycogen synthase (prevents glucose storage).
- Activates glycogen phosphorylase (promotes glycogen breakdown).
- Effect in liver only: The liver releases glucose into the bloodstream to raise blood glucose levels. Muscle cells do not respond to glucagon as their glycogen stores are for their own use.
Epinephrine (Adrenaline - Stress Hormone):
- Released during stressful situations (e.g., fight-or-flight response).
- Causes phosphorylation of enzymes, similar to glucagon's effect.
- Activates glycogen phosphorylase.
- Inactivates glycogen synthase.
- Effect in both liver and muscle: Both tissues respond by breaking down glycogen—muscle for its immediate energy (e.g., for physical activity), and liver to provide glucose to the brain and other tissues during stress.

2. Glycolysis and Fermentation

Glycolysis is the metabolic pathway that breaks down glucose, generating ATP. It is regulated at its irreversible steps.

Glycolysis Net Reaction

One molecule of glucose (6 carbons) is broken down into two molecules of pyruvate (3 carbons each).
This is an oxidative process, producing: $ \text{Glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 ext{P}i \rightarrow 2 \text{Pyruvate} + 2 ext{NADH} + 2 ext{H}^+ + 2 \text{ATP} + 2 \text{H}2\text{O} $
Other monosaccharides (e.g., fructose, galactose, mannose) can also be channeled into glycolysis.
The products (ATP, NADH, pyruvate) are all precursors or directly usable for further ATP production.

Regulation of Glycolysis

The pathway is regulated at its three irreversible steps. While the first and third steps (catabolized by hexokinase and phosphofructokinase, respectively) are early control points, the last step (pyruvate kinase) is also regulated because it is the first irreversible step of gluconeogenesis in reverse.

Phosphofructokinase (PFK) (Step 3 enzyme): A key regulatory enzyme.
- Inhibitors:
  - ATP: High ATP levels indicate sufficient energy, so glycolysis is inhibited to conserve glucose. This can lead to a build-up of glucose-6-phosphate, which can then be shunted towards glycogen synthesis.
  - Citrate: An intermediate of the citric acid cycle, high citrate also signals ample energy supply, inhibiting glycolysis.
- Activators:
  - AMP, ADP, Pi: High concentrations signal low energy status, activating PFK to produce more ATP.
  - Fructose-2,6-bisphosphate: This compound, whose concentration increases due to insulin signaling, acts as a potent activator of PFK, promoting glycolysis when blood glucose is high.
Insulin's dual role: Insulin simultaneously promotes glucose storage (glycogenesis) and immediate glucose utilization (glycolysis). This is analogous to managing a paycheck: some money is spent on immediate needs, and some is saved. Cells use glucose for immediate ATP needs while storing excess as glycogen.

Fermentation[

Fermentation is a metabolic process that produces ATP without net oxidation or reduction of molecules. Its primary role is to regenerate $ext{NAD}^+$ (consumed in glycolysis) when oxygen is scarce, allowing glycolysis to continue.

Oxygen Dependence: Normally, NADH transfers its electrons to the electron transport chain, with oxygen as the final electron acceptor. Without oxygen, NADH accumulates, and $ext{NAD}^+$ is depleted, halting glycolysis.
Lactate Fermentation (e.g., Muscle Cells during intense exercise):
- Pyruvate is reduced to lactate.
- NADH is oxidized back to $ext{NAD}^+$ .
- Net reaction: $ \text{Glucose} + 2 ext{ADP} + 2 ext{P}i \rightarrow 2 \text{Lactate} + 2 \text{ATP} + 2 \text{H}2\text{O} $
Ethanol Fermentation (e.g., Yeast):
- Pyruvate is decarboxylated to acetaldehyde, then reduced to ethanol.
- NADH is oxidized back to $ext{NAD}^+$ .
- Net reaction: $ \text{Glucose} + 2 ext{ADP} + 2 ext{P}i \rightarrow 2 \text{Ethanol} + 2 \text{CO}2 + 2 \text{ATP} + 2 \text{H}_2\text{O} $
Definition: Fermentation encompasses the entire pathway from glucose to the final reduced product (e.g., lactate or ethanol) that allows continuous ATP production without oxygen. Brain cells do not rely on fermentation due to insufficient ATP yield.

3. Gluconeogenesis

Gluconeogenesis is an anabolic process where the liver (and to a lesser extent, kidneys) synthesizes glucose from non-carbohydrate precursors, primarily pyruvate, lactate, and certain amino acids. Its main function is to maintain blood glucose levels, especially during prolonged fasting or exercise when glycogen stores are depleted.

Energetic challenge: Directly reversing glycolysis would be energetically unfavorable ( $ext{positive} riangle ext{G}$ ). To overcome this, gluconeogenesis employs different enzymes for the irreversible steps of glycolysis, performing different reactions.
Energy Cost: Gluconeogenesis requires a significant energy input, consuming six ATP equivalents for every molecule of glucose synthesized:
- $ 2 \text{Pyruvate} + 4 \text{ATP} + 2 \text{GTP} + 2 \text{NADH} + 2 \text{H}^+ + 4 \text{H}2\text{O} \rightarrow \text{Glucose} + 4 \text{ADP} + 2 \text{GDP} + 6 \text{P}i + 2 \text{NAD}^+ $
- GTP is energetically equivalent to ATP.
Liver's role: The liver synthesizes glucose and exports it into the bloodstream for use by other tissues (e.g., the brain), as liver cells typically do not use this newly synthesized glucose for their own ATP needs.

Regulation of Gluconeogenesis

Regulation occurs at the irreversible steps, often in inverse relation to glycolysis regulation.

Fructose-2,6-bisphosphate: When insulin is high, this compound's concentration rises, and it acts as an inhibitor of fructose-1,6-bisphosphatase (a key enzyme in gluconeogenesis). This prevents glucose synthesis when blood glucose is already high.
AMP: High AMP levels (indicating low cellular ATP) inhibit gluconeogenesis. This is a protective mechanism for the liver cell: it must prioritize its own energy needs before expending large amounts of ATP to produce glucose for the rest of the body.

4. Pentose Phosphate Pathway (PPP)

The pentose phosphate pathway is primarily involved in biosynthesis.

Main products: NADPH (an electron donor for anabolic reactions) and ribose-5-phosphate (a precursor for nucleotide synthesis).
Two operational modes (versions depend on carbon fate):
- Version A (NADPH production with glycolytic carbon recycling):
  - Glucose-6-phosphate is oxidized to CO2 (generating NADPH).
  - The remaining carbons are converted to intermediates (e.g., fructose-6-phosphate, glyceraldehyde-3-phosphate) that feed back into the glycolysis pathway.
  - This mode allows cells to produce NADPH while still generating some ATP via glycolysis.
- Version B (NADPH and Nucleotide Synthesis):
  - Glucose-6-phosphate is oxidized to CO2 (generating NADPH).
  - The remaining carbons are used to produce ribose-5-phosphate, which is directly used for synthesizing nucleotides (DNA, RNA).
  - This mode prioritizes biosynthesis over ATP production from these specific carbons.
Flexibility: Cells can adjust the flux through these two versions of the PPP based on their specific needs for NADPH and nucleotide precursors. Typically, both versions operate simultaneously, with their relative contribution shifting according to cellular demand.