Metabolism and Glycogen Flashcards
Metabolism
Metabolism is a complex network of biochemical reactions that sustain life, involving the transformation of molecules, energy production, and waste elimination.
Even simple organisms have a vast number of reactions (around 4,000 in cells), showcasing the intricate coordination required for cellular function.
Humans, as multicellular organisms with cell specialization, exhibit a diverse array of metabolic reactions across different tissues, ages, developmental stages, and during wound healing processes.
Tissue-specific metabolic profiles contribute to the unique functions of organs.
Metabolic reactions share similarities across organisms due to common evolutionary origins and adherence to thermodynamic laws, illustrating the conservation of fundamental biochemical principles.
Metabolic Pathways
Metabolic pathways are interconnected sequences of enzymatic reactions that transform specific molecules, known as metabolites.
Enzymes are crucial for reactions to occur at rates that sustain life; without them, reactions would proceed too slowly to support cellular processes.
Enzyme deficiencies can lead to metabolic disorders.
Metabolic pathways are somewhat artificial delineations, helping to organize and understand the complex network of chemical reactions within a cell.
They are interconnected and regulated.
It's important to view metabolic pathways as an interconnected network where metabolites are diverted based on cellular needs.
Flux through these pathways is dynamically regulated.
Compartmentalization
In eukaryotes, metabolic reactions are compartmentalized within the cell, enhancing efficiency and regulation.
Organelles provide distinct environments for specific pathways.
This compartmentalization allows for better regulation and efficiency of metabolic processes by separating pathways and preventing interference.
For example, the mitochondrial matrix provides an ideal environment for the TCA cycle.
Glycolysis (glucose breakdown) occurs in the cytosol, breaking down glucose into pyruvate.
The pentose phosphate pathway (nucleotide creation) also takes place in the cytosol, generating NADPH and precursors for nucleotide synthesis.
The citric acid cycle (TCA cycle), electron transport, oxidative phosphorylation, and fatty acid oxidation occur in the mitochondria, facilitating energy production and lipid metabolism.
Mitochondrial dysfunction is implicated in various diseases.
Transport systems are necessary to move intermediates between cellular compartments, such as the movement of glycolysis end products into the TCA cycle.
These facilitate the coordinated function of metabolic pathways.
These transport systems often involve specific membrane proteins that facilitate the passage of molecules across organelle membranes.
Examples include the malate-aspartate shuttle and the glycerol-3-phosphate shuttle.
Enzymes, especially those catalyzing nonreversible reactions, act as control points, regulating the flow of metabolites through pathways.
These enzymes often serve as targets for drugs.
These control points are often subject to allosteric regulation, where the binding of a molecule to the enzyme affects its activity.
Feedback inhibition is a common regulatory mechanism.
Types of Metabolic Reactions
Metabolic reactions can be categorized as degradative (catabolic) or synthetic (anabolic).
Degradative reactions break down molecules, often generating energy (exergonic reactions).
This energy is harnessed to drive cellular processes.
During catabolism, large molecules such as polysaccharides, lipids, and proteins are broken down into smaller, simpler molecules like glucose, fatty acids, and amino acids.
Energy released is captured by molecules like ATP (adenosine triphosphate), serving as the primary energy currency of the cell.
ATP serves as the primary energy currency of the cell, providing the energy needed for various cellular processes, including muscle contraction, nerve impulse transmission, and biosynthesis.
Anabolic reactions build larger molecules and require energy input as well as reduction (electrons being fed into the reaction via coenzymes like NADH and FADH2).
These are vital for growth, repair, and maintenance.
Anabolic pathways often utilize ATP and reducing power (NADH, FADH2) generated during catabolism to synthesize complex molecules from simpler precursors.
This process is essential for building cellular components.
Catabolic pathways break down carbohydrates, fats, and proteins into energy core products, often passing through common intermediates like acetyl CoA.
These pathways converge to extract energy from diverse fuel sources.
Carbohydrates: Forms and Functions
Carbohydrates are a major fuel source in catabolic reactions and are abundant in diets, providing energy for daily activities.
They come in various forms:
Polysaccharides are long strings of sugar molecules (polymers) that can be branched, serving as storage forms of glucose.
Examples include starch, glycogen, and cellulose.
Starch is a primary energy storage form in plants, while glycogen serves as the main energy storage in animals.
Cellulose is a structural component of plant cell walls.
Oligosaccharides are smaller bunches of subunits, often attached to proteins and lipids on cell surfaces.
They are often found on the surface of cells, where they play a role in cell recognition and signaling.
Glycoproteins and glycolipids are examples of these structures.
Disaccharides consist of two saccharide units linked together and are commonly found in foods.
Common examples are sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar).
Monosaccharides are single sugar subunits (e.g., glucose), the simplest form of carbohydrates.
Monosaccharides like glucose, fructose, and galactose are the building blocks of more complex carbohydrates.
They are readily absorbed and utilized for energy.
Glucose is a crucial fuel molecule for many tissues, particularly the brain and red blood cells.
It is the primary source of energy for these cells.
Red blood cells primarily use glucose (glycolysis) as they lack mitochondria and cannot break down fats.
This makes them dependent on glucose for ATP production.
Brain tissue preferentially utilizes glucose unless under extreme conditions (starvation) where it can use ketone bodies.
This preference ensures a constant supply of energy for neuronal function.
During prolonged starvation, the brain adapts to use ketone bodies as an alternative fuel source to spare glucose for other tissues.
This adaptation helps to maintain brain function during periods of limited glucose availability.
Glucose can also be used as a skeleton to synthesize neurotransmitters, highlighting its role beyond energy provision.
It serves as a precursor for various signaling molecules.
Blood glucose concentration is maintained relatively stable (4-8 millimolar, often cited as 5 millimolar).
Hormonal regulation is crucial for maintaining this balance.
This tight regulation is crucial for maintaining proper cellular function and preventing complications associated with hyperglycemia or hypoglycemia.
Dysregulation can lead to diabetes and other metabolic disorders.
Glucose has a linear form () with chiral carbon centers, allowing for different arrangements in 3D space, influencing its interaction with enzymes.
Glucose exists as enantiomers (non-superimposable mirror images): L-form and D-form.
D-glucose is the physiologically relevant form.
Metabolism primarily deals with D-glucose, as enzymes are stereospecific for this isomer.
This specificity is critical for efficient metabolism.
Glucose is obtained from diet, glycogen storage, or gluconeogenesis, ensuring a constant supply to meet the body's energy needs.
These sources work together to maintain glucose homeostasis.
Glucose Cyclization
Linear glucose can cyclize to form a six-membered ring called pyranose (glucopyranose), the predominant form in solution.
The anomeric carbon (carbonyl group-holding carbon) allows for stereoisomers like alpha and beta anomers, influencing its reactivity and enzymatic processing.
The alpha and beta anomers differ in the orientation of the hydroxyl group attached to the anomeric carbon.
This difference impacts their susceptibility to enzymatic action.
The term "alpha-D-glucopyranose" refers to the alpha anomeric form of D-glucose when cyclized, a specific stereoisomer.
Five-membered rings are referred to as furanoses, which are less common in glucose but prevalent in fructose.
Carbohydrate Uptake
Digestion of carbohydrates begins in the mouth with salivary alpha-amylase, which randomly breaks alpha 1-4 glycosidic bonds (bonds between sugar molecules).
This initiates the breakdown of complex carbohydrates.
The enzyme stops working in the acidic environment of the stomach, halting carbohydrate digestion temporarily.
In the duodenum, pancreatic alpha-amylase continues breaking down carbohydrates after the acidic contents are neutralized by bicarbonate, ensuring optimal enzymatic activity.
In the jejunum and small intestine, disaccharidases on the luminal brush border break down carbohydrates into monosaccharides, facilitating their absorption into the bloodstream.
Examples include sucrase, lactase, and maltase, which break down sucrose, lactose, and maltose, respectively.
Deficiencies in these enzymes can lead to digestive disorders like lactose intolerance.
Intestinal cells use a secondary active transport system to uptake glucose, coupling its transport with the movement of sodium ions.
Energy is used to pump out sodium, creating an electrochemical gradient.
Sodium flows back in via a co-transporter, bringing glucose into the cell against its concentration gradient.
This allows for efficient glucose absorption even when its concentration is low.
The sodium-glucose cotransporter 1 (SGLT1) is a key protein involved in this process.
Mutations in SGLT1 can cause glucose-galactose malabsorption.
Other cell types use passive GLUT transporters, which are tissue-specific (e.g., GLUT4 in neurons, GLUT2 in the liver), to move glucose down its concentration gradient, maintaining cellular glucose levels.
GLUT4 is insulin-dependent and plays a crucial role in insulin-stimulated glucose uptake in muscle and adipose tissue.
Insulin resistance impairs GLUT4 translocation, contributing to type 2 diabetes.
Summary of Biochemistry and Metabolism
Biochemistry and metabolism encompass all chemical reactions in the body, essential for life.
Catabolic metabolism breaks things down, while anabolic metabolism builds things up, maintaining a dynamic balance.
Glucose is a key component of carbohydrate metabolism, serving as a central fuel molecule.
Glycogen: Storage and Homeostasis
Glycogen is a storage system evolved to ensure a consistent supply of glucose within the body, particularly during periods of fasting or increased energy demand.
Glycogen serves as a readily available source of glucose during periods of increased energy demand or when dietary glucose is limited.
It helps maintain blood glucose homeostasis.
It addresses the fact that glucose intake from the diet isn't constant, providing a buffer against fluctuations in blood glucose levels.
Gluconeogenesis is another system that comes into play after glycogen stores are depleted, synthesizing glucose from non-carbohydrate precursors.
Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors such as lactate, glycerol, and amino acids.
It is essential for maintaining blood glucose levels during prolonged fasting.
Glycogen is found in the liver and skeletal muscle, each with different roles in glucose homeostasis and energy provision.
The liver acts as the body's fuel dump, storing glycogen for most other organs, maintaining blood glucose levels.
Liver glycogen helps maintain blood glucose homeostasis by releasing glucose into the bloodstream when needed.
This is particularly important during overnight fasting.
The liver doesn't prioritize using glucose, saving it to provide to the rest of the body, ensuring a stable supply for other tissues.
The liver's role is to store glycogen and mobilize it to form glucose in order to support blood glucose levels, preventing hypoglycemia.
Concentration in liver is around 10 % fresh weight, reflecting its significant role in glycogen storage.
Skeletal muscle stores glycogen (about 400 grams overall) for ATP creation during muscle contraction, supporting physical activity.
Muscle glycogen provides a readily available source of glucose for energy during exercise or other physical activity.
It is primarily used by the muscle itself.
Problems with glycogen storage result in glycogen storage diseases that can manifest in the liver or muscles, leading to various clinical symptoms.
Glycogen storage diseases are a group of inherited metabolic disorders characterized by abnormal glycogen metabolism, leading to glycogen accumulation in various tissues.
These diseases can cause hepatomegaly, muscle weakness, and hypoglycemia.
Glycogen is a polysaccharide with many glucose subunits linked together in a specific, branched structure, optimizing glucose storage and release.
It forms black granules inside the cell, visible under electron microscopy, providing a visual representation of glycogen stores.
A single glycogen molecule can be very large (up to Daltons), highlighting its capacity to store substantial amounts of glucose.
Glycogen is a highly branched, very dense sugar molecule, maximizing the number of non-reducing ends for efficient glucose mobilization.
If all the glucose in hepatocyte glycogen molecules were broken down into separate glucose units, the concentration could reach 0.4 molar, causing osmotic problems.
This illustrates the importance of glycogen's compact structure.
When wrapped up in glycogen, however, the osmotic pressure is much lower (about 10 nanomolar), preventing cellular damage from osmotic stress.
Glycogen Bonding
Up to 10,000 glucose units are clipped together with alpha 1-4 glycosidic bonds and alpha 1-6 glycosidic bonds at branch points, creating its characteristic structure.
Alpha 1-4 bonds are on the main chain, while alpha 1-6 bonds create branches, enhancing solubility and accessibility.
Glycogen has reducing and non-reducing ends, where enzymes work on non-reducing ends, facilitating rapid glucose release.
The non-reducing ends are where glucose units are added or removed during glycogen synthesis and breakdown.
This allows for quick mobilization of glucose when needed.
The branching architecture strikes a balance, providing enough branches for enzymes to release glucose, but not so many that the molecule becomes too dense, optimizing glucose mobilization.
Breaking the main chain is rapid, but managing a branch is slow, hence the branch length optimizes glucose release.
At the same time, there are enough branches to get enzymes in, but not that there are too few branches such that cutting down the number if available glucose decreases.
Glycogenesis and Glycogenolysis
Glycogenesis is the creation of glycogen, the process of synthesizing glycogen from glucose molecules.
Glycogenesis involves several enzymes, including glycogen synthase, which is responsible for adding glucose units to the growing glycogen chain.
Insulin stimulates glycogenesis in the liver and muscle.
Glycogenolysis is the breakdown of glycogen, releasing glucose molecules for energy production or blood glucose maintenance.
Glycogenolysis is primarily regulated by glycogen phosphorylase, which breaks down glycogen into glucose-1-phosphate.
Glucagon and epinephrine stimulate glycogenolysis.
A common feature is glucose-1-phosphate, an initial substrate for synthesis and a main breakdown product from glycogenolysis, interlinking these processes.
Phosphoglucomutase creates glucose-1-phosphate, converting glucose-6-phosphate to glucose-1-phosphate and vice versa, facilitating its entry into glycogenesis or glycolysis.
Enzymes like glycogen phosphorylase can't work too close to a branch point and stop working, leaving four sugar units next to the branch point called a limit dextrin.
This structure requires a debranching enzyme for further degradation.
Glycogen Synthesis
Glycogen synthesis requires a primer protein called glycogenin to begin, initiating the formation of a new glycogen molecule.
This requires an activated form of glucose called UDP-glucose (uridine diphosphate glucose), formed from glucose-1-phosphate and uridine triphosphate, providing the energy for glycogen synthesis.
After the primer is set up, glycogen synthase, which is the main responsible enzyme, clips UDP-glucose together with 1-4 glycosidic bonds into a long chain, elongating the glycogen molecule.
A transferase breaks the main chain, moves a chunk of it, and sticks it on a branch point, creating the branched structure of glycogen.
Glucose-6-phosphate is converted to glucose-1-phosphate by phosphoglucomutase, an interconversion step.
Uridine triphosphate attaches to glucose via UDP-glucose pyrophosphorylase, releasing pyrophosphate, activating glucose for glycogen synthesis.
This UDP-glucose attaches to glycogenin, and then glycogen synthase makes the alpha 1-4 bonds, creating a chain, elongating the glycogen molecule.
The branching enzyme trims a six-saccharide unit and attaches them to a branch point via an alpha 1-6 bond, at which point two chains can be worked on by glycogen synthase, creating branches to optimize synthesis and breakdown.
Glycogen Breakdown
Glycogenolysis involves glycogen phosphorylase, the key enzyme in glycogen breakdown.
This enzyme binds glycogen about 30 angstroms away from its active site, positioning it for efficient cleavage of glucose units.
The enzyme runs into a branch point and can't continue due to the protein structure; it leaves a four-carbohydrate unit limit, requiring a debranching enzyme.
The enzyme utilizes inorganic phosphate to cleave off glycogen, producing glucose-1-phosphate.
The debranching enzyme moves three sugar units to the end of the main chain and cleaves the remaining glucose, removing the branch point.
About 10% of the glucose in glycogen is released without phosphate by the debranching enzyme.
The debranching enzyme has two activities: transferase and glucosidase.
Connection to Other Metabolic Processes
Glycogen synthesis and breakdown create glucose-1-phosphate as well as pure glucose, linking glycogen metabolism to other pathways.
These are transformed into glucose-6-phosphate, a key intermediate that can enter multiple metabolic pathways.
Glucose-6-phosphate can undergo glycolysis, be stripped of the phosphate group and exported, be used in the hexose monophosphate shunt for nucleotide creation, converted to pyruvate and then to acetyl CoA for the TCA cycle or be subjected to anaerobic respiration to create lactate, etc.
It is a central metabolic hub.
Regulation of Glucose Metabolism
Maintaining glucose metabolism is significant to prevent hypoglycemia (too little glucose) or hyperglycemia and glycosuria (too much glucose), ensuring proper cellular function and preventing disease.
Synthesis occurs in a bowel-fed state in the liver (post-meal), storing excess glucose.
Breakdown will occur two or three hours after a meal (fasting), maintaining blood glucose levels.
Glycogen will be synthesized in muscle during periods of rest and will be broken down during periods of exercise, providing energy during activity.
Regulation occurs on different levels involving hormonal regulation by insulin, glucagon, and adrenaline (epinephrine), which act through cell surface receptors to modulate enzyme activities as well as allosteric regulation where metabolites directly influence enzyme activity.
Hormones act through cell receptors, triggering intercellular signaling cascades.
Second messengers like cAMP and calcium mediate these effects.
Adrenaline and glucagon (glucogenic hormones) signal the need for glucose release and work through cyclic AMP, activating glycogenolysis.
Cyclic AMP (cAMP) acts as a secondary messenger, activating protein kinases that regulate glycogen metabolism.
It initiates a cascade of phosphorylation events.
Receptors that are receptive to any hormone depend upon which tissue you are dealing with.
Glucagon just effects the liver, increasing glycogenolysis in the liver.
Adrenaline effects skeletal muscle, as well as liver and muscle, increasing glycogenolysis in both tissues.
Cyclic AMP will end up interacting with the regulatory subunits of protein kinase A, activating it.
This protein kinase then phosphorylates target enzymes.
Once active, it can phosphorylate glycogen synthase, but will actually inactivate it.
Glycogen synthase is inactivated when phosphorylated.
Phosphorylation of glycogen synthase decreases its activity, inhibiting glycogen synthesis.
This is a key regulatory step.
With loads of glucose about, there'll be lots of insulin, which will impact enzymes such as protein phosphatase, which will remove the phosphate from your glycogen synthase, which will then allow glycogen synthesis to store glucose.
This phosphatase is activated by insulin.
Protein phosphatase dephosphorylates glycogen synthase, increasing its activity and promoting glycogen synthesis, countering the effects of glucagon and epinephrine.
Insulin will also encourage this enzyme here to break down your secondary messenger cyclic AMP such that inactivation of your synthase cannot occur, maintaining low cAMP levels.
By reducing cAMP levels, insulin inhibits the activation of protein kinase A, thereby promoting glycogen synthesis.
This further enhances glucose storage.
Allosteric Regulators and Glycogen Phosphorylase
Glycogen synthase is activated by the presence of lots of glucose-6-phosphate, signaling high glucose availability.
Production of cyclic AMP will activate proteinkinase A, which will put a phosphate group on glycogen kinase, which will activate it.
This then phosphorylates glycogen phosphorylase which will activate it, facilitating the glycogen breakdown.
If you've just had a steak with large amounts of fuel present, systems says, okay, no need to break down glycogen which prompts insulin will then activate and impact enzymes in the reservoir which realize that there is no need to degrade glycogen which prompts it to try proteins, which will allow for synthesis to occur.
Glycogen phosphorylase regulation can be activated, and deactivated by proteinkinase A or proteophosphatase 1, allowing for fine-tuned control.
Regulators come in different forms and are present at different sites: i.e. different platelet sites on protein.
Allosteric regulators bind at different sites to exert their influence.
Examples of regulators include: glycogen six phosphate, ATP, AMP, etc.
These small molecules modulate enzyme activity depending on energy status.
If energy is low, AMP( adenosine monophosphate) will rise in concentration, and will activate glycogen phosphorylase, stimulating glycogen breakdown
Neurostimulation will prompt glycogen breakdown in muscle cells. In essence, rate-limiting enzymes and irreversible steps are critical for metabolic regulation.
Glycolysis
In glycolysis, the rate-limiting enzyme is phosphofructokinase-1 (PFK-1), which catalyzes the committed step of phosphorylating fructose-6-phosphate to fructose-1,6-bisphosphate.
PFK-1 is regulated by ATP, AMP, citrate, and fructose-2,6-bisphosphate.
The irreversible steps are catalyzed by hexokinase, PFK-1, and pyruvate kinase.
Hexokinase phosphorylates glucose to glucose-6-phosphate, trapping it inside the cell.
Pyruvate kinase converts phosphoenolpyruvate (PEP) to pyruvate, generating ATP.
Gluconeogenesis
In gluconeogenesis, the rate-limiting enzyme is fructose-1,6-bisphosphatase (FBPase-1), which dephosphorylates fructose-1,6-bisphosphate to fructose-6-phosphate, bypassing the PFK-1 step.
FBPase-1 is inhibited by AMP and fructose-2,6-bisphosphate.
The irreversible steps are catalyzed by pyruvate carboxylase, phosphoenolpyruvate carboxykinase (PEPCK), FBPase-1, and glucose-6-phosphatase.
Pyruvate carboxylase converts pyruvate to oxaloacetate, requiring ATP and biotin.
PEPCK converts oxaloacetate to phosphoenolpyruvate (PEP), requiring GTP.
Glucose-6-phosphatase dephosphorylates glucose-6-phosphate to glucose in the endoplasmic reticulum of liver and kidney cells.
Citric Acid Cycle (TCA Cycle)
In the citric acid cycle (TCA cycle), the rate-limiting enzyme is isocitrate dehydrogenase, which catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing .
It is regulated by ATP, ADP, and .
The irreversible steps are catalyzed by citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.
Citrate synthase condenses acetyl-CoA with oxaloacetate to form citrate.
Alpha-ketoglutarate dehydrogenase converts alpha-ketoglutarate to succinyl-CoA, producing and releasing .
Glycogenesis Regulation
In glycogenesis, the rate-limiting enzyme is glycogen synthase, which adds UDP-glucose to glycogen, forming alpha-1,4-glycosidic bonds.
It is regulated by glucose-6-phosphate, insulin, and phosphorylation.
The irreversible step is the formation of UDP-glucose by UDP-glucose pyrophosphorylase.
UDP-glucose pyrophosphorylase converts glucose-1-phosphate and UTP to UDP-glucose and pyrophosphate.
Glycogenolysis Regulation
In glycogenolysis, the rate-limiting enzyme is glycogen phosphorylase, which cleaves alpha-1,4-glycosidic bonds in glycogen, releasing glucose-1-phosphate.
It is regulated by glucagon, epinephrine, insulin, AMP, ATP, and calcium ions.
The irreversible step is the phosphorolytic cleavage of glycogen by glycogen phosphorylase, which breaks down glycogen into glucose-1-phosphate.
Fatty Acid Synthesis
In fatty acid synthesis, the rate-limiting enzyme is acetyl-CoA carboxylase (ACC), which carboxylates acetyl-CoA to form malonyl-CoA.
It is regulated by citrate, palmitoyl-CoA, AMP, and insulin.
The irreversible step is the carboxylation of acetyl-CoA by ACC, which converts acetyl-CoA to malonyl-CoA, requiring ATP and biotin.
Beta-Oxidation
In beta-oxidation, the rate-limiting enzyme is carnitine palmitoyltransferase I (CPT-I).
CPT-I transfers fatty acyl-CoA from the cytosol