Tema 6

Metabolism of Carbohydrates: An Overview

Introduction to Carbohydrate Metabolism

• The session begins with a review of carbohydrate metabolism, emphasizing the importance of understanding digestion as the first step in this process.

• Carbohydrates are broken down by various enzymes in the gastrointestinal tract into simpler forms like monosaccharides (galactose, glucose, fructose).

Absorption Mechanisms

• Monosaccharides are absorbed at the intestinal epithelium through specific transport proteins; SGLT1 and SGLT2 facilitate active transport of glucose.

• Other transporters such as GLUT1, GLUT2, GLUT3, and GLUT4 allow for facilitated diffusion of monosaccharides without energy expenditure.

Transporter Characteristics

• Glucose transporters (GLUT1, 2, 3) exhibit tissue-dependent expression; GLUT4 is particularly significant due to its insulin regulation.

• These transporters can move glucose bidirectionally and also handle other sugars like lactose and fructose.

Sodium-Glucose Cotransport

• Glucose enters epithelial cells via sodium-glucose cotransport (symporter), which relies on sodium gradients maintained by the Na+/K+ ATPase pump.

• This pump ensures optimal ion balance by transporting three sodium ions into the cell while moving two potassium ions out.

Electrochemical Gradients and Conformational Changes

• The electrochemical gradient favors sodium influx from extracellular space into epithelial cells, facilitating glucose binding to transport proteins.

• A conformational change in the transporter occurs upon binding both sodium and glucose, allowing for their entry into the cell.

Passive Transport to Bloodstream

• Once inside epithelial cells, glucose is transported passively into the bloodstream via GLUT2 when concentrations favor movement from high to low.

Glycolysis: The Pathway of Energy Production

Overview of Glycolysis

• After absorption, glucose undergoes glycolysis—a metabolic pathway that converts it into pyruvate while generating ATP.

• Glycolysis is nearly universal across organisms and plays a central role in energy generation and metabolite production.

Phases of Glycolysis

• The glycolytic pathway consists of ten steps divided into three phases: an energy investment phase where ATP is consumed; cleavage phase where six-carbon glucose splits into two three-carbon molecules; and an energy generation phase producing ATP.

Key Steps:

1. Energy Investment Phase: Two ATP molecules are used to phosphorylate intermediates.

• This sets up subsequent reactions leading to sugar breakdown.

2. Cleavage Phase: Glucose splits into two trioses (three-carbon sugars).

• These trioses are then phosphorylated for further processing.

3. Energy Generation Phase: High-energy compounds formed yield four ATP molecules from ADP through substrate-level phosphorylation.

• The net result from one molecule of glucose yields two pyruvate molecules and a total gain of two ATP molecules.

Glycolysis: Key Steps and Regulation

Preparatory Phase of Glycolysis

• The initial step involves the phosphorylation of glucose to glucose 6-phosphate, requiring one ATP molecule. This reaction is catalyzed by the enzyme hexokinase.

• Glucose 6-phosphate undergoes isomerization by phosphoglucoisomerase, followed by a second ATP investment to form fructose 1,6-bisphosphate.

• Fructose 1,6-bisphosphate is cleaved by aldolase into two molecules: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

• DHAP is converted into another G3P molecule through the action of triose phosphate isomerase, resulting in two G3P molecules.

Energy Generation Phase

• Glyceraldehyde 3-phosphate undergoes oxidation and phosphorylation via glyceraldehyde 3-phosphate dehydrogenase, producing two NADH molecules that provide reducing power for the citric acid cycle.

• The product at this stage is 1,3-bisphosphoglycerate which generates ATP through substrate-level phosphorylation via phosphoglycerate kinase.

• Phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate; enolase then dehydrates it to form phosphoenolpyruvate (PEP).

• Pyruvate kinase catalyzes the final step where PEP is converted to pyruvate, yielding an additional two ATP molecules. This phase is known as the payoff phase.

Regulation of Glycolysis

• Glycolysis regulation occurs at various levels; key regulators include ATP levels affecting hexokinase activity and AMP/ADP activating it.

• Pyruvate kinase serves as a rate-limiting step regulated positively by AMP and negatively by ATP, alanine, and acetyl-CoA.

• Covalent modification can also regulate pyruvate kinase activity through glucagon-induced activation or inactivation via phosphorylation.

Sources Feeding into Glycolysis

• Other carbohydrate degradation pathways contribute substrates for glycolysis; sucrose breakdown releases glucose while lactose degradation does so via lactase.

• Glycogen degradation produces glucose 1-phosphate which can be converted to glucose 6-phosphate for glycolytic entry through phosphoglucomutase.

Gluconeogenesis Overview

• Gluconeogenesis converts non-carbohydrate precursors like glucogenic amino acids, lactate from Cori cycle, and glycerol from lipid metabolism into glucose or glycogen.

• It allows for new glucose production or carbohydrate biosynthesis from three or four-carbon precursors.

Gluconeogenesis: Key Processes and Enzymes

Overview of Gluconeogenesis

• Gluconeogenesis involves the conversion of non-carbohydrate sources into glucose, primarily utilizing amino acids from muscle proteins and other precursors like propionate.

• The liver and kidneys are the main organs responsible for gluconeogenesis, with the liver being the primary site for this metabolic pathway.

Pathway Details

• Pyruvate is converted to oxaloacetate, which enters gluconeogenesis to ultimately produce glucose. This process also links to energy metabolism through the Krebs cycle.

• Acetyl-CoA acts as a positive regulator for oxaloacetate formation, while excess acetyl-CoA inhibits pyruvate dehydrogenase activity.

Metabolic Intermediates

• Various intermediates in the citric acid cycle (Krebs cycle), such as oxaloacetate, support glucose synthesis by facilitating reactions that lead to pyruvate formation.

• Three irreversible biochemical reactions in glycolysis—catalyzed by hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase—are bypassed in gluconeogenesis using specific enzymes.

Energy Investment in Gluconeogenesis

• The gluconeogenic pathway requires an energy investment; it consumes ATP and GTP during various steps to synthesize glucose from pyruvate or alanine.

• The first step involves converting two molecules of pyruvate into two molecules of oxaloacetate via pyruvate carboxylase, requiring ATP and CO2.

Key Enzymatic Steps

• Following oxaloacetate formation, malate is produced and transported out of mitochondria where it is converted back into oxaloacetate before forming phosphoenolpyruvate (PEP).

• Fructose 1,6-bisphosphate is generated from PEP through several enzymatic steps involving ATP investment. This compound is then converted into fructose 6-phosphate by fructose 1,6-bisphosphatase.

Final Steps in Glucose Production

• Fructose 6-phosphate undergoes further transformation into glucose 6-phosphate before being converted into free glucose by glucose 6-phosphatase through hydrolysis.

• Essential enzymes include pyruvate carboxylase (mitochondrial), phosphoenolpyruvate carboxykinase (cytosolic), fructose 1,6-bisphosphatase (cytosolic), and glucose 6-phosphatase (endoplasmic reticulum).

Energy Balance Analysis

• Overall energy expenditure includes two molecules of pyruvate, four ATP equivalents (ATP/GTP), reducing power from NADH, resulting in one molecule of glucose along with other byproducts like ADP and phosphate groups.

Metabolic Pathways and Energy Recycling

Lactate Metabolism and Gluconeogenesis

• Lactate is metabolized to pyruvate, which plays a role in the glucose-alanine cycle, generating alanine for pyruvate synthesis.

• In plants, CO2 fixation and intermediate metabolite formation occur through the glyoxylate cycle.

• The conversion of pyruvate to glucose requires an investment of four ATP molecules and two GTP molecules.

Liver-Muscle Interaction

• The liver synthesizes glucose from lactate, which is then sent to muscles for energy production; this creates a recycling system for energy.

• Alanine can be converted back into pyruvate in the liver with an investment of six ATP molecules, facilitating communication between liver and muscle metabolism.

Lipid Metabolism

• Fatty acids and triglycerides serve as energy sources; beta-oxidation in mitochondria produces acetyl-CoA that enters the citric acid cycle.

• Triglycerides provide glycerol for gluconeogenesis by forming glycerol phosphate, linking lipid metabolism with carbohydrate pathways.

Glycogenolysis Overview

• Glycogen breakdown releases glucose 1-phosphate and free glucose, crucial for maintaining blood sugar levels during fasting.

• Glycogen storage varies: approximately 72g in the liver (4% of total carbohydrates), 245g in muscle (7%), with only about 10g extracellularly.

Regulation of Glycogen Metabolism

• Glycogen phosphorylase breaks down glycogen into glucose 1-phosphate; this process is tightly regulated by hormones like insulin and glucagon.

• Insulin negatively regulates glycogen phosphorylase activation while glucagon promotes its activity, illustrating hormonal control over energy release.

Metabolism of Glycogen: Key Regulatory Mechanisms

Activation of Glycogen Phosphorylase

• The second messenger facilitates the activation of glycogen phosphorylase, which is crucial for glycogen metabolism.

• UDP-glucose, a modified form of glucose, plays a significant role in elongating the glycogen chain by associating with glucose molecules.

Glycogen Structure and Enzymatic Action

• Glycogen chains are structured with alpha 1,4 linkages; phosphorylase generates non-reducing ends through strategic cuts, creating branches that release internal glucose molecules.

Role of cAMP in Cellular Signaling

• Cyclic AMP (cAMP), an important second messenger, initiates signaling at the cell membrane via receptor proteins that activate G-protein coupled receptors.

• Upon ligand binding, G-proteins dissociate into active components that promote GDP phosphorylation and activate adenylate cyclase to produce cAMP.

Activation of Protein Kinase A (PKA)

• cAMP binds to PKA's regulatory subunits, leading to its activation. Active PKA phosphorylates various proteins including transcription factors like CREB.

• Phosphorylated CREB associates with DNA response elements to regulate genes involved in glycogen synthesis and degradation.

PI3K Pathway and Calcium Release

• Another regulatory pathway involves phosphoinositide 3-kinase (PI3K), which phosphorylates phosphatidylinositol leading to the production of secondary messengers like IP3 and diacylglycerol.

• IP3 triggers calcium release from the endoplasmic reticulum while diacylglycerol activates protein kinase C (PKC), enhancing glycogen breakdown in liver cells.

Hormonal Regulation of Glycogen Metabolism

• Hormones such as glucagon and adrenaline stimulate glycogen degradation through pathways activated by cyclic AMP; insulin counteracts this by activating protein phosphatases that inhibit glycogen phosphorylase.

• The balance between these hormones regulates glucose availability from glycogen stores versus its storage.

Summary of Hormonal Effects on Glycogenesis

• Adrenaline activates adenylate cyclase leading to increased cAMP levels which enhance enzyme activity related to glycogenesis while inhibiting phosphatases that would otherwise deactivate these enzymes.

• This hormonal interplay ensures efficient regulation between glycogenesis and glycolysis based on cellular energy needs.

This structured overview captures key insights into the metabolic processes governing glycogen utilization and regulation within cells.

Importance of Glucose in Human Body

Role of Glucose

• Glucose is the primary energy source for the human body, essential for tissues like the brain and skeletal muscle that rely heavily on it.

Blood Sugar Regulation

• The body maintains blood glucose levels between 70 to 100 mg/dL during fasting. After digestion, carbohydrates break down into monosaccharides such as glucose, lactose, and galactose.

Absorption Mechanisms

• Monosaccharides are absorbed in the small intestine via membrane transporters:

• SG LT1 transporter introduces glucose with sodium into intestinal cells.

• GLUT transporter allows glucose passage from intestines to blood.

• GLUT4 is insulin-sensitive, facilitating glucose entry into skeletal muscle and adipose tissue.

Metabolic Pathways

• Once inside cells, glucose undergoes glycolysis in the cytoplasm, converting one molecule of glucose into two pyruvate molecules while producing ATP (the immediate energy currency).

Hormonal Regulation