Carbohydrate Metabolism: Module 7

Carbohydrate Metabolism Overview: Module 7

• Focusing is on the metabolic pathways involved in energy production from carbohydrate metabolism in this module.
• Sugars consumed or derived from glycogen stored in the muscles and liver will be examined.
• Glucose metabolism is the primary focus, but other sugars are also metabolized through similar pathways.
• Glycolysis, pyruvate dehydrogenase reaction, and the citric acid cycle sequentially break down glucose.
• These pathways are tightly regulated to control carbohydrate breakdown as needed.
• Glycolysis can occur anaerobically, resulting in lactate production in humans, or ethanol in yeast.
• Gluconeogenesis is the de novo synthesis of glucose that occurs when glycogen stores are low.
• The brain uses this process as it prefers glucose as fuel.
• Excess dietary carbohydrates get stored as glycogen in the muscle and liver unless glycogen stores are full.
• Excessive carbohydrates are then converted to fats for storage.
• Both glycogen degradation and synthesis are highly regulated.
• The pentose phosphate pathway is an alternative route for glucose breakdown.
• NADPH is generated for reductive biosynthetic processes, such as fat synthesis.
• Ribose-5-phosphate is used for nucleotide biosynthesis.

Learning Objectives

• Identify the metabolic fates of glucose-6-phosphate and how they are related.
• Explain glycogen synthesis and degradation in general terms.
• Describe the net reactions of glycolysis and gluconeogenesis, including the stoichiometry.
• Identify the metabolic conditions that favor glycogen synthesis, glycogen degradation, glycolysis, and gluconeogenesis without memorizing individual regulators.
• Provide the purpose of fermentation, the circumstances in which it is used, and the products of the main fermentation pathways.
• Explain the pentose phosphate pathway, and identify the fates of carbon atoms entering this pathway.
• Describe how opposing pathways (glycolysis and gluconeogenesis) can be energetically favorable.

Glycogen Structure & Function

• Vertebrates store glucose as glycogen.
• Glycogen’s structure includes a single reducing end and several non-reducing ends.
• Alpha-1,4 bonds in glycogen provide for linear linkage of glucose subunits.
• Alpha-1,6 bonds in glycogen create the branch points in the structure.
• During glycogen degradation, glucose residues get sequentially removed from non-reducing ends.
• This simultaneous process provides a rapid surge of glucose when it is needed.

Glycogen Regulation

• Glycogen phosphorylase and glycogen synthase regulate glycogen breakdown and synthesis.
• Glycogen phosphorylase catalyzes the phosphorolysis of glycogen via the addition of inorganic phosphate (Pi).
• Glycogen phosphorylase action releases a glucose residue as glucose-1-phosphate.
• The phosphoglucomutase enzyme then converts glucose-1-phosphate into glucose-6-phosphate.
• Glycogen synthase catalyzes glycogen synthesis from glucose-1-phosphate, using UTP for energy.

Metabolic Regulation Details

• Enzymatic reactions in metabolism are tightly regulated to prevent futile cycling, such as glycogen breakdown happening with glycogen synthesis.
• The goal is to match energy supply (ATP) with energy demand, similar to economics balancing supply and demand.

Allosteric Regulation

• Glycogen synthase is allosterically activated by high concentrations of glucose-6-phosphate.
• High concentrations of glucose-6-phosphate signal that there is an abundance of carbohydrate for glycogen storage.
• Glycogen phosphorylase is allosterically activated by high AMP concentrations.
• High AMP concentrations show a low energy status, therefore signaling more glucose residues for glycolysis to produce ATP.
• High ATP concentrations inhibit glycogen phosphorylase.
• This results in ample energy with no further need for substrate breakdown.

Covalent Regulation

• Glycogen synthase and glycogen phosphorylase are regulated covalently by phosphorylation.
• Covalent addition of a phosphate group acts as a switch that can turn an enzyme on or off.
• Glycogen phosphorylase is activated through phosphorylation by a kinase.
• Glycogen phosphorylase is inactivated through phosphate group removal by a phosphatase.
• Glycogen synthase is inactivated through phosphorylation, and activated through phosphate group removal.
• Hormonal signals regulate the phosphorylation and dephosphorylation by kinases and phosphatases.
• Insulin stimulates dephosphorylation by the phosphatase.
• Glucagon and epinephrine stimulate phosphorylation by the kinase.

Insulin's Role in Glycogen Metabolism

• Insulin release responds to elevated glucose levels by signaling glucose uptake to be used for energy or stored for later use.
• After insulin binds to its receptor, it results in glycogen synthase activation and glycogen phosphorylase inactivation due to an abundance of available glucose.

Glucagon's Role in Glycogen Metabolism

• Glucagon is counter-regulatory to insulin.
• Glucagon is released when blood glucose levels drop, signaling the liver to release more glucose.
• Glucagon inactivates glycogen synthase and activates glycogen phosphorylase.

Glycolysis Details

• Once glucose enters a cell from the bloodstream, it can be oxidized to provide useable energy in the form of ATP.
• Glycolysis uses 10 reactions to convert glucose into pyruvate.
• The 6-carbon glucose molecule splits into two 3-carbon pyruvate molecules.
• Glycolysis produces 2 ATP and 2 NADH molecules.
• Pyruvate can be further oxidized in the mitochondria; NADH can also be oxidized by the electron transport chain.
• Fructose and galactose can also feed into glycolysis to be oxidized.

Values for Glycolysis Reactions

• Glycolysis is highly regulated with Enzymes catalyzing reactions 1 and 3 being targets for regulation.
• Reactions 1 and 3 have a large change in free energy and are considered irreversible.

Control of Flux Through Glycolysis

• The enzyme hexokinase catalyzes step 1 of glycolysis.
• Hexokinase is inhibited by its product, glucose-6-phosphate, via end-product inhibition.
• Abundant glucose-6-phosphate signals sufficient substrate for glycolysis to proceed, preventing additional glucose breakdown.
• The enzyme phosphofructokinase catalyzes step 3, and is inhibited by high ATP and citrate concentrations.
• ATP as an inhibitor signals sufficient energy levels.
• Citrate, as the first product in the tricarboxylic acid cycle, signals sufficient substrate.
• Phosphofructokinase is stimulated by high concentrations of AMP and ADP, which signal low ATP.
• Activity is upregulated by fructose-2,6-bisphosphate.

Fermentation

• Glycolysis reduces NAD+ to NADH, therefore NAD+ must be regenerated for the pathway to continue.
• When oxygen is present, the electron transport chain regenerates NAD+
• When oxygen is absent, NAD+ is regenerated by other reactions.
• Fermentation is a pathway that produces ATP with no net carbon oxidation when O2 is absent.

Formation of Lactate

• One form of fermentation is the formation of lactate, which is common in microorganisms and some eukaryotic cells, such as muscle cells during exercise.
• Lactate dehydrogenase oxidizes NADH, reduces pyruvate to lactate, and regenerates NAD+, which allows glycolysis to continue.

Formation of Ethanol

• Ethanol is produced in yeast rather than lactate during fermentation, also regenerating NAD+ and allowing glycolysis to continue.

Fermentation Net Reactions

• Fermentation produces 2 ATP molecules to meet cellular ATP demand when oxygen is absent.

Glycogen and Glycolysis Summary

• Excess glucose is stored as glycogen, which is regulated allosterically and by enzyme phosphorylation, responding to hormones.
• In glycolysis, glucose gets oxidized to pyruvate, creating 2 ATP and 2 NADH.
• Glycolysis is regulated allosterically and by glucose availability.
• Fermentation yields ATP without O2, without any net carbon oxidation.

Gluconeogenesis Details

• Gluconeogenesis is the de novo glucose synthesis when glycogen sources are low.
• Gluconeogenesis occurs mostly in the liver, and a bit in the kidneys.
• The brain constantly needs and prefers glucose.
• Precursors like amino acids, some citric acid cycle intermediates, and lactate via pyruvate can be converted to oxaloacetate, and then to glucose, through gluconeogenesis.

Gluconeogenesis Energetics

• Gluconeogenesis isn't a simple reversal of glycolysis.
• It bypasses the three non-equilibrium reactions of glycolysis reactions, which have large changes in free energy.
• Gluconeogenesis requires an energy input because glycolysis generates energy input.
• Different substrate concentrations and the use of non-equilibrium reactions helps to drive gluconeogenesis.

Gluconeogenesis Net Reaction

• Gluconeogenesis’s net reaction, beginning from pyruvate, requires 4 ATP plus 2 GTP, which is higher than the 2 ATP from glycolysis.
• 2 NADH are also needed.
• The glucose made can then be exported from the liver to meet the body’s demands.

Comparing Glycolysis and Gluconeogenesis

• Gluconeogenesis is not a direct reversal of glycolysis.
• While both pathways share near-equilibrium enzymes, steps 1, 3, and 10 are catalyzed by different enzymes.
• Glucokinase
• Phosphofructokinase
• Pyruvate Kinase
• These must be bypassed using:
• Glucose-6-phosphatase
• Fructose bisphosphatase
• PEP carboxykinase
• Pyruvate carboxylase.

Control of Flux of Gluconeogenesis

• Gluconeogenesis’s flux is regulated reciprocally to that in glycolysis, which prevents cycling without purpose.
• Fructose bisphosphatase is a key regulation point.
• Fructose bisphosphatase is inhibited by high AMP and fructose-2,6-bisphosphate concentrations.

F6P and F1,6P Cycle Regulation

• Phosphofructokinase gets activated by AMP and fructose-2,6-bisphosphate.
• When the cell’s energy state is high (high ATP, low AMP), gluconeogenesis gets stimulated.
• When the cell energy state is low (low ATP, high ADP and AMP), glycolysis is stimulated.

Pentose Phosphate Pathway Specifics

• The pentose phosphate pathway is an alternate to break down glucose, producing NADPH for reductive biosynthetic processes (like synthesizing fat), as well as providing ribose-5-phosphate for making nucleotides.
• Glucose-6-phosphate is oxidized to generate NADPH and carbon dioxide.
• When nucleotide synthesis is required, the pentose phosphate pathway will divert carbons to ribose-5-phosphate production.

Pentose Phosphate Pathway

• Three glucose-6-phosphate + 6 NADP are converted into 2 Fructose-6-phosphate + glyceraldehyde-3-phosphate (+3 CO2 + 6 NADPH).
• Carbons can also be metabolized to two fructose-6-phosphate and one glyceraldehyde-3-phosphate from the three original glucose-6-phosphate molecules.
• Intermediates then feed into glycolysis and get metabolized for energy.

Review of Gluconeogenesis and PPP

• The liver can use gluconeogenesis to make its own glucose if glycogen stores run out by spending 6 ATPs per glucose.
• Gluconeogenesis is not the opposite/reverse of glycolysis.
• Gluconeogenesis’is regulation is opposite to glycolysis’s regulation.
• Pentose phosphate pathway can produce both NADPH and ribose-5-phosphate.