Metabolic Regulation and Hormonal Signaling: Study Guide

Metabolic Regulation

Chemical Messengers

Metabolic regulation in biochemistry is largely controlled by chemical messengers, which include hormones that communicate signals between cells and tissues to coordinate physiological processes.

• Endocrine: Endocrine cells secrete hormones directly into the bloodstream. These hormones act on specific target cells that may be far away from the site of secretion. Examples: Insulin, glucagon, cortisol.

• Paracrine: Paracrine signals are secreted from cells and act locally on nearby cells of a different type. Examples: Neurotransmitters, growth factors.

• Autocrine: Autocrine signals act on the same cell that secreted them or on nearby cells of the same type. Examples: Some immune cell signals.

Note: Some signals can function in more than one manner.

Cell Signaling

Cell signaling begins with the release of a chemical messenger and involves a series of steps that transmit the signal to the target cell, resulting in a physiological response.

• Messenger will diffuse or is transported in the blood/extracellular fluids.

• Messenger binds to a specific receptor either on a target cell membrane (for hydrophilic messengers) or diffuses through the lipid bilayer (for hydrophobic messengers).

• Binding to the receptor elicits a response.

• Signal can be terminated after the response.

• Hydrophobic chemical messengers are specific for intracellular receptors (e.g., steroid hormones).

• Hydrophilic chemical messengers are specific for plasma membrane receptors (e.g., peptide hormones).

Example: Insulin binds to tyrosine kinases.

Intracellular Signaling

Receptors as Transcription Factors (Steroid Hormone Signaling)

Steroid hormones such as cortisol typically function by binding to intracellular receptors that act as transcription factors, directly influencing gene expression.

• Cortisol is released from the adrenal cortex and diffuses into the bloodstream.

• Cortisol is hydrophobic and is transported attached to serum albumin and steroid hormone binding globulin.

• Cortisol diffuses into the plasma membrane of the cell.

• Binding to its cortisol receptor (intracellular, located in the cytosol) induces a conformational change.

• The receptor-ligand complex translocates to the nucleus.

• It binds to a portion of DNA called the hormone response element (HRE) or glucocorticoid response element (GRE).

• This results in either the activation (increase) or repression (decrease) of gene transcription, depending on the location of the GRE.

Example: Cortisol regulates metabolism by altering gene expression in target tissues.

Membrane Bound Receptors

G-Protein-Coupled Receptor Cascade (Glucagon Signaling)

Membrane-bound receptors often use intracellular second messengers to transmit signals. The G-protein-coupled receptor (GPCR) pathway is a classic example, especially in glucagon signaling.

• Hormone binds to a 7-helix GPCR, causing a conformational change that activates a G-protein on the cytosolic side of the membrane.

• GDP is exchanged for GTP on the Gα subunit, activating it.

• Activated Gα stimulates adenylyl cyclase, which converts ATP to cyclic AMP (cAMP).

• cAMP activates Protein Kinase A (PKA), which phosphorylates target proteins.

• Phosphodiesterases can degrade cAMP and terminate the signal.

• Different Gα subunits have different effects:

  • Gs — increases cAMP

  • Gi — decreases cAMP

  • Gq — increases phospholipase C activity

Example: Glucagon signaling in the liver increases blood glucose by activating glycogen breakdown.

Insulin Signaling

Tyrosine Kinase Receptor Pathway

Insulin signaling is mediated by receptor tyrosine kinases, which amplify signals through kinase cascades.

• Insulin receptor dimerizes upon ligand binding.

• Phosphorylation of the receptor leads to phosphorylation of Insulin Receptor Substrate (IRS).

• IRS activates downstream kinases:

  • Phosphoinositide Kinase 1 (PDK1)

  • Protein Kinase B (PKB, also known as Akt)

  • Grb2 — activates MAPK cascade

• These proteins have SH2 domains and bind to different sites on IRS.

Example: Insulin promotes glucose uptake and storage in muscle and adipose tissue.

Phosphatidylinositol Metabolism

Second Messenger Generation

Phosphatidylinositol metabolism provides a connection between hormone receptor and intracellular calcium signaling.

• Phospholipase C cleaves PIP2 to yield two second messengers:

  • Inositol-1,4,5-trisphosphate (IP3) — enhances Ca2+ release from the endoplasmic reticulum.

  • Diacylglycerol (DAG) — activates Protein Kinase C.

• Kinases catalyze sequential transfer of phosphate from ATP to hydroxyl groups at positions 5 & 4 of the inositol ring of phosphatidylinositol, yielding phosphatidylinositol-1,4,5-bisphosphate (PIP2).

Example: Hormone signaling via Gq-coupled receptors.

Metabolism in the Fed State

Hormone Levels and Glucose Uptake

In the fed state, hormone levels and tissue-specific glucose uptake mechanisms are summarized below.

Hormones	Levels
Insulin	Elevated
Glucagon	LOW
Cortisol	LOW
Epinephrine	LOW

Tissue	Fuel Used	Mechanism of Glucose Uptake
Liver	Dietary GLUCOSE	GLUT 2
Skeletal muscle	Dietary GLUCOSE	GLUT 4 (insulin-sensitive)
Red blood cells	Dietary GLUCOSE	GLUT 1
Brain	Dietary GLUCOSE	GLUT 1 and 3

Metabolism in the Fasted State

Hormone Levels and Fuel Utilization

In the fasted state, hormone levels shift and tissues utilize different fuels to maintain energy balance.

Hormones	Levels
Insulin	LOW
Glucagon	Elevated
Cortisol	Elevated
Epinephrine	Elevated

Tissue	Fuel Used	Pathway Providing Fuel
Liver	Free fatty acids	Lipolysis
Skeletal muscle	Free fatty acids	Lipolysis
Red blood cells	GLUCOSE	Hepatic gluconeogenesis and glycogenolysis
Brain	GLUCOSE	Hepatic gluconeogenesis and glycogenolysis (no storage in this tissue)
Adipose	Free fatty acids (not much energy needed in this tissue)	Lipolysis

Pathways Activated/Enhanced by Each Hormone

The following table summarizes the major metabolic pathways activated or enhanced by each hormone in key tissues:

	Insulin (anabolic)	Glucagon (catabolic)	Epinephrine (catabolic)	Cortisol (catabolic)
Liver	Glycolysis, TCA, Glycogen synthesis, Fatty acid synthesis, Protein synthesis	Glycogenolysis, Gluconeogenesis, Lipolysis	Glycogenolysis, Gluconeogenesis (via cAMP pathway), Fatty acid oxidation	Enhances gluconeogenesis through transcriptional changes, increases amino acid catabolism, promotes protein breakdown
Skeletal muscle	Glucose uptake, Protein synthesis, Glycogen synthesis through GLUT4	No impact	Glycogenolysis, Glycolysis via cAMP pathway	Promotes protein breakdown to provide amino acids for gluconeogenesis in the liver
Adipose	Triacylglycerol synthesis, Glucose uptake through GLUT4	Lipolysis	Lipolysis	Lipolysis

Additional info: These tables summarize the major roles of each hormone, but each hormone may have additional functions not listed here.