13.3: Gluconeogenesis

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Learning Goals 

(Learning goals written by Claude, Sonnet 4.6, Anthropic)

Overview and Metabolic Logic of Gluconeogenesis

• Identify the principal gluconeogenic substrates (lactate, glycerol, and glucogenic amino acids), explain where gluconeogenesis occurs in mammals, and describe the physiological conditions that activate the pathway.
• Explain why gluconeogenesis cannot simply be the reverse of glycolysis, and identify the three irreversible glycolytic reactions that require dedicated bypass enzymes, stating the energetic cost of the bypass from phosphoenolpyruvate to glucose-6-phosphate.

Bypass Reactions and Their Mechanisms

• Describe the two-step conversion of pyruvate to phosphoenolpyruvate, explaining the role of biotin as a mobile carboxyl carrier in pyruvate carboxylase — including the carboxylation of biotin in the BC domain and the carboxylation of pyruvate's enolate form in the CT domain — and the subsequent GTP-dependent decarboxylation-phosphorylation catalyzed by PEPCK.
• Explain why oxaloacetate cannot be directly exported from the mitochondrial matrix, and trace the sequence of reactions in the malate-aspartate shuttle that allows its carbon skeleton to reach the cytosol for PEPCK-catalyzed PEP synthesis.
• Describe the mechanisms of fructose-1,6-bisphosphatase and glucose-6-phosphatase, including their metal ion requirements, the structural basis of allosteric regulation (T and R states) in FBPase-1, and the significance of the ER-luminal location of glucose-6-phosphatase.

Regulation and Physiological Integration

• Explain how fructose-1,6-bisphosphatase is reciprocally regulated relative to phosphofructokinase-1 by AMP, ATP, and fructose-2,6-bisphosphate, and describe how this reciprocal regulation coordinates the opposing fluxes of glycolysis and gluconeogenesis.
• Connect gluconeogenesis to the Cori cycle by explaining how lactate produced by anaerobic muscle metabolism is shuttled to the liver, converted to pyruvate, and used to regenerate blood glucose, with the energetic cost of gluconeogenesis met by hepatic oxidative metabolism.

Introduction

Gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate carbon substrates, such as lactate, glycerol, and glucogenic amino acids. It is one of the two main mechanisms humans and many other animals use to keep blood glucose levels from dropping too low (hypoglycemia). The other means of maintaining blood glucose levels is glycogen degradation (glycogenolysis). Gluconeogenesis is ubiquitous in plants, animals, fungi, bacteria, and other microorganisms. In animals, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the renal cortex. This process occurs during fasting, starvation, low-carbohydrate diets, or intense exercise and is highly endergonic. For example, the pathway from phosphoenolpyruvate to glucose-6-phosphate requires 6 ATP molecules. Gluconeogenesis is often associated with ketosis. Gluconeogenesis is also a therapeutic target for type II diabetes, such as metformin, which inhibits glucose formation and stimulates cell uptake.

Lactate is transported back to the liver, where it is converted to pyruvate by the Cori cycle via lactate dehydrogenase. Pyruvate, the first designated substrate of the gluconeogenic pathway, can then be used to generate glucose (Figure \(\PageIndex{1}\)). All citric acid cycle intermediates, through conversion to oxaloacetate, amino acids other than lysine or leucine, and glycerol can also function as substrates for gluconeogenesis. Transamination or deamination of amino acids facilitates the entry of their carbon skeleton into the cycle directly (as pyruvate or oxaloacetate) or indirectly via the citric acid cycle. Glycerol, a part of the triacylglycerol molecule, can be used in gluconeogenesis.

Gluconeogenesis is a pathway consisting of eleven enzyme-catalyzed reactions. Depending on the substrate being used, the pathway can begin in the mitochondria or cytoplasm. Many of the reactions are the reversible steps found in glycolysis (Figure \(\PageIndex{1}\)). In humans, gluconeogenesis is restricted to the liver and, to a lesser extent, the kidney.

Figure\(\PageIndex{1}\): Gluconeognesis

Bypass I (reverse of step 10 in glycolysis): Pyruvate to Phosphoenolpyruvate

The conversion of pyruvate to phosphoenolpyruvate involves two enzymatic steps, with oxaloacetate as the intermediate. In all species, the formation of oxaloacetate from pyruvate and any other TCA cycle intermediates is restricted to the mitochondrion, and the enzymes that convert PEP to glucose are found in the cytosol. The location of the enzyme that links these two parts of gluconeogenesis by converting oxaloacetate to PEP, PEP carboxykinase, varies by species: it can be found entirely within the mitochondria, entirely within the cytosol, or evenly distributed between the two, as in humans. Dedicated transport proteins move PEP across the mitochondrial membrane; however, no such proteins exist for oxaloacetate. Therefore, in species that lack intra-mitochondrial PEP, oxaloacetate must be converted into malate or aspartate, exported from the mitochondrion, and converted back into oxaloacetate to allow gluconeogenesis to continue.

Here is the net reaction: Pyruvate + ATP + HCO₃^- + GTP → PEP + ADP + GDP + CO₂, ΔG^o' = +0.2 kcal/mol (0.8 kJ/mol)

Figure\(\PageIndex{2}\) shows the overall reaction for converting pyruvate to phosphoenol pyruvate.

 

Figure \(\PageIndex{2}\): Overall reaction for the conversion of pyruvate to phosphoenol pyruvate

In systems that produce oxaloacetate in the mitochondria and then need to transport it to the cytosol to be converted into phosphoenolpyruvate, three enzymes are needed for the process: pyruvate carboxylase (PC), located in the mitochondrial matrix, aspartate aminotransferase (AAT), located in the matrix and the cytosol, and the phosphoenolpyruvate carboxykinase (PCK), located in the cytosol. Oxaloacetate and aspartate are intermediate compounds formed in the process.

In the first reaction, pyruvate carboxylase converts pyruvate into oxaloacetate, based on the following reaction:

Pyruvate carboxylase: pyruvate + HCO₃^- + ATP → oxaloacetate + ADP + P_i

The pyruvate carboxylase enzyme requires biotin as a cofactor. It has two major enzymatic functions: 1) carbon dioxide is fixed or attached to biotin, and 2) a carbon transferase activity, moving the fixed carbon dioxide to pyruvate to create oxaloacetate. Figure (\PageIndex{3}\) shows the biotin cofactor. Biotin is a water-soluble vitamin (D-biotin or vitamin B7) and a cofactor in enzymatic reactions involving carboxylation (or carbon fixation reactions). It is part of the vitamin B2 complex and is an essential vitamin for mammals—a deficiency results in dermatitis, hair loss, and neurologic symptoms.

Figure (\PageIndex{3}\): Chemical Structure of Biotin. Biotin is shown in several renderings: (A) line structure, (B) stick model, (C) ball-and-stick model, and (D) space-filling model. Figure from: Biosynthesis

The biotin cofactor forms an amide linkage with the pyruvate carboxylase enzyme at a lysine residue (Figure (\PageIndex{4}\)). This creates a flexible linker region within pyruvate carboxylase, capable of positioning the biotin cofactor within the enzyme's different catalytic domains to express catalytic activity. Attaching biotin to the enzyme requires ATP energy.

 

Figure (\PageIndex{4}\) Attachment of Biotin Cofactor to Lysine Residue in Pyruvate Carboxylase. Image from: Biosynthesis

The pyruvate carboxylase enzyme is a tetramer that contains four functional protein subunits, as shown in Figure \(\PageIndex{5}\). The BCCP flexible arm can extend into the biotin carboxylation (BC) domain, where biotin is first carboxylated. Then, the BCCP shifts to the carboxyl transferase (CT) domain, where the fixed carbon dioxide can be transferred to pyruvate, forming oxaloacetate.

Figure \(\PageIndex{5}\) Structure of the Pyruvate Carboxylase Enzyme. Pyruvate carboxylase is a tetramer composed of four functional protein subunits. Two subunits are shown in color, with the other two indicated in gray at the back of the structure. (a) shows the space-filling model, while (b) shows the major domains as a cartoon graphic. Each subunit contains a biotin carboxylase (BC) domain shown in blue, a carboxytransferase (CT) domain shown in yellow, and the biotin-carboxyl carrier protein (BCCP) domain shown in red and green.  Figure modified from Liu, Y., et al (2018) Nat Commun 9:1384

In the first part of the reaction, the biotin cofactor is carboxylated using bicarbonate as a substrate, with ATP driving the formation of a high-energy intermediate.  (Remember, there is no such thing as a "high energy" bond.) Thus, biotin acts as an intermediate carrier of the carboxy group that will be added to pyruvate during the formation of oxaloacetate. This occurs in the biotin carboxylase (BC) domain, where biotin is bound to the biotin carboxyl carrier protein (BCCP) domain. The carboxyl group is then transferred to pyruvate to form oxaloacetate in the carboxyl transferase (CT) domain. Let's break up the enzyme mechanism from Rhizobium etli into three figures.

Figure\(\PageIndex{6}\) below shows the first steps in the carboxylation of biotin in the biotin carboxylase domain, using bicarbonate as a substrate. In Figure\(\PageIndex{6}\), you can see that oxygen from bicarbonate mediates nucleophilic attack on the outer phosphate group of the ATP molecule, forming the carbonyl-phosphate intermediate + ADP. The carbonyl-phosphate decomposes to release carbon dioxide and phosphate. The negatively charged phosphate activates the biotin cofactor to enable a nucleophilic attack on the carbon of the carbon dioxide, producing the carboxybiotin intermediate, shown in the upper part of Figure\(\PageIndex{7}\).

Figure \(\PageIndex{6}\): The Carboxylation of Biotin during the Pyruvate Carboxylase Reaction Mechanism. Bicarbonate, shown in blue, enters the enzyme's active site and reacts with the terminal phosphate of ATP, forming a phosphorylated intermediate. Two Mg²⁺ cofactors stabilize this reaction. The carboxyphosphate intermediate decomposes, releasing carbon dioxide and phosphate.

Image modified from Ribeiro AJM et al. (2017), Nucleic Acids Res, 46, D618-D623. Mechanism and Catalytic Site Atlas (M-CSA): a database of enzyme reaction mechanisms and active sites. DOI:10.1093/nar/gkx1012. PMID:29106569. https://www.ebi.ac.uk/thornton-srv/m-csa/entry/223/. Creative Commons Attribution 4.0 International (CC BY 4.0) License.

Figure\(\PageIndex{7}\): Carboxylation of biotin in pyruvate carboxylase. The upper diagram shows the actual carboxylation of biotin, which occurs in the BC domain. The carboxybiotin intermediate shifts on the flexible arm and enters the carboxytransferase (CT) domain. Pyruvate enters the active site, initiating the carboxylation reaction that forms oxaloacetate. Ribeiro, ibid

Carboxyltransferase activity is located in the CT domain of the enzyme. It requires the formation of a carbanion intermediate on the pyruvate molecule, making it reactive enough to allow the addition of the carboxyl group. A general base from the enzyme removes a proton from the C3 position of pyruvate, forming an enol intermediate. As the electrons on the oxygen atom of the pyruvate molecule reform the carbonyl group, the electrons from the carbon-carbon double bond shift onto the carbon, forming a reactive carbanion intermediate. The carboxybiotinyl complex swings into the CT domain, providing the carboxyl group that will be incorporated into pyruvate. The carbanion intermediate mediates a nucleophilic attack on the carboxyl carbon from the BCCP complex, and carbon dioxide is transferred to the pyruvate, creating oxaloacetate. The biotin cofactor is restored by the donation of a proton from a basic residue in the enzyme. This also restores the pyruvate carboxylase enzyme for another round of activity.

Figure\(\PageIndex{8}\) below shows the carboxylation of the enolate form of pyruvate to form oxaloacetate.

Figure\(\PageIndex{8}\): Carboxylation of the enolate form of pyruvate to form oxaloacetate by pyruvate carboxylase. Ribeiro et al, ibid

Figure\(\PageIndex{9}\) below shows an interactive iCn3D model of the biotin-dependent multifunctional enzyme pyruvate carboxylase from Rhizobium etli (2QF7)

Figure\(\PageIndex{9}\): Biotin-dependent multifunctional enzyme pyruvate carboxylase from Rhizobium etli (2QF7). The enzyme is a homotetramer with C2 symmetry, but only one subunit chain is shown for clarity. The biotin carboxylase domain (BC) is shown in cyan, the carboxytransferase domain (CT) in yellow, a C-terminal biotin carboxyl carrier protein (BCCP) domain (disconnected in parts) in magenta, and the allosteric domain in green. AGS in the cyan domain is a phosphothiophosphoric acid-adenylate ester, an ATP analog. It is shown interacting with the 2 Mg²⁺ ions, and it is represented in sticks and labeled. Acetyl-CoA and a nonhydrolyzable analog, ethyl-CoA, are allosteric activators. CoASH (ethyl group not shown) is illustrated in spacefill bound to the green allosteric domain. Acetyl-CoA binding induces a conformational change that shortens the distance between the two active sites on the BT (cyan) and CT (yellow) domains. Biotin, not shown, is tethered in the BCCP domain, which transfers biotin between the two catalytic domains, not within a single monomer (as shown above), but between different monomers in the homo 4-mer.

(Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...7g91JmzJMad4r9

Figure\(\PageIndex{10}\) shows the flexible nature of the biotin flexible arm of the enzyme during the catalytic process.

 

Figure\(\PageIndex{10}\) The Catalytic Activity of Pyruvate Carboxylase. The Biotin-Carboxyl Carrier Protein Domain (BCCP) is a flexible arm of the protein structure that can swing between the two catalytic domains during the reaction. As shown in the diagram, the BCCP domain of subunit 1 will be fixed with carbon dioxide at the biotin carboxylase domain from subunit 1 (BC1). The BCCP will then translocate to the carboxyltransferase domain from subunit 2 (CT2), creating one molecule of oxaloacetate and resetting the biotin cofactor. The BCCP will then enter the BC2 domain, where carbon fixation occurs. BCCP then shifts to the CT1 domain, creating a second oxaloacetate molecule and resetting the biotin. While this happens with the BCCP from subunit 1, the other BCCP arms from subunits 2, 3, and 4 are also active. Thus, one full sequence of the enzyme activity will produce eight oxaloacetate molecules.  Image from: Liu, Y., et al (2018) Nat Commun 9:1384

The next reaction in the process converts oxaloacetate to phosphoenolpyruvate via phosphoenolpyruvate carboxykinase (PEPCK). However, the major human PEPCK enzyme, PCK1, resides in the cell's cytoplasm. Thus, the oxaloacetate produced in the mitochondrial matrix by pyruvate carboxylase must be transported into the cytosol. However, there are no oxaloacetate transporters that can mediate direct transfer. To leave the mitochondrial matrix, oxaloacetate must first be converted to aspartate by the aspartate aminotransferase enzyme (AAT). It can then be transported to the intermembrane space of the mitochondria through an antiporter that transports one molecule of aspartate out of the matrix and one molecule of glutamate into the matrix Figure\(\PageIndex{11}\). Once in the intermembrane space, the aspartate can pass freely through a pore in the outer mitochondrial membrane. When it reaches the cytoplasm, aspartate is reconverted to oxaloacetate by cytoplasmic aspartate aminotransferase (AAT). Oxaloacetate can then serve as a substrate for the phosphoenolpyruvate carboxykinase (PEPCK) enzyme.

Figure\(\PageIndex{11}\) Conversion of Oxaloacetate to Aspartate and Transfer to the Cytoplasm.  Image modified from SMART Servier Medical Art

The glutamate/aspartate transporter has additional complexities. It cannot function on its own. It requires the coordinated functioning of the malate/alpha-ketoglutarate antiporter. Together, these antiporters are known as the Malate-Aspartate Shuttle System.

The Malate-Aspartate Shuttle System is dependent on the functioning of two enzymatic processes. The first is the aspartate aminotransferase indicated more simplistically in Figure\(\PageIndex{11}\). The aspartate aminotransferase enzyme can utilize glutamate as an amine donor to generate aspartate from oxaloacetate. Alpha-ketoglutarate is also formed in this process. This enzyme can also work in the reverse reaction to produce glutamate and oxaloacetate, depending on substrate concentrations and other regulatory mechanisms. In a different reaction catalyzed by malate dehydrogenase, oxaloacetate can be reduced to malate using a molecule of NADH as the electron donor.

Figure\(\PageIndex{12}\) Chemical Reactions of the Malate-Aspartate Shuttle System

Figure modified from Son, H.F. and Kim, J-J. (2016) PLoS One 10.1371

The malate dehydrogenase enzyme is expressed at high levels in the cytoplasm of liver cells and in the mitochondrial matrix. This enzyme is a component of the Krebs cycle, where it mediates the formation of oxaloacetate in the last step of the cycle. Within the cytoplasm, it predominantly converts oxaloacetate to malate. The malate can then be shuttled into the matrix of the mitochondria through the malate/alpha-ketoglutarate antiporter (Figure\(\PageIndex{13}\)). In this antiporter, malate moves into the mitochondrial matrix while alpha-ketoglutarate moves into the intermembrane space (and subsequently into the cell's cytosol). Together with aspartate, it can be used by cytoplasmic aspartate aminotransferase to produce glutamate and oxaloacetate. This oxaloacetate pool is then utilized in the gluconeogenic pathway when PEPCK is active. The glutamate generated from this reaction is transported back into the matrix of the mitochondria through the aspartate/glutamate antiporter. The aspartate pool in the matrix of the mitochondria is supplied by the reaction of the aspartate aminotransferase enzyme, which completes the reverse reaction from the one seen in the cytoplasm. In the matrix, aspartate aminotransferase uses glutamate and oxaloacetate as substrates to generate aspartate and alpha-ketoglutarate. This enables the transport of aspartate and glutamate via their respective antiporters. In the gluconeogenic pathway, oxaloacetate is produced in the mitochondrial matrix. Oxaloacetate is then converted to aspartate and transported across the inner membrane, where it can subsequently be converted back into oxaloacetate and used for glucose production.

Figure\(\PageIndex{13}\): The Malate-Aspartate Shuttle System.

Once oxaloacetate has been effectively transported via the Malate-Aspartate Shuttle into the cytoplasm, it is converted to phosphoenolpyruvate (PEP) by the PEP carboxykinase (PEPCK). The overall reaction mediated by PEPCK is:

PEP carboxykinase (PEPCK): oxaloacetate + GTP → phosphoenol pyruvate + GDP

PEPCK is classified as a lyase enzyme and exists in two isozymes: a cytoplasmic PEPCK and a mitochondrial PEPCK. The cytoplasmic form is predominantly used in the gluconeogenic pathway and requires the Malate-Aspartate Shuttle. However, small amounts of PEP can be made directly by the mitochondrial PEPCK and transported across the mitochondrial membrane. The conversion of oxaloacetate to phosphoenolpyruvate by PEPCK mediates the removal of carbon dioxide and the addition of a phosphate group. GTP serves as the phosphate donor during this process, and magnesium ions act as cofactors. Figure \(\PageIndex{14}\) shows the reaction mechanism for the human PEP carboxykinase.

Figure \(\PageIndex{14}\): Reaction mechanism for human PEP carboxykinase (PEPCK)

Figure\(\PageIndex{15}\) below shows an interactive iCn3D model of phosphoenolpyruvate carboxykinase (PEPK) with a bound PEP and GTP-competitive inhibitor (1NHX).

Figure \(\PageIndex{15}\): Phosphoenolpyruvate carboxykinase (PEPK) with a bound PEP and GTP-competitive inhibitor (1NHX). PEP is shown in colored sticks. The competitive inhibitor is shown in colored spacefill. The active site residues from the human PEPCK mechanism are shown as labeled sticks. (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...QFcTssCJbuW2K7

Phosphatase Reactions of Gluconeogenesis (reverse of steps 1 and 3 in glycolysis):

The last two unique enzymes in the gluconeogenic pathway are both phosphatases. Fructose 1,6-bisphosphatase converts fructose 1,6-bisphosphate to fructose 6-phosphate. The phosphoglucoisomerase enzyme in glycolysis can convert fructose-6-phosphate back into glucose-6-phosphate. A unique glucose-6-phosphatase enzyme will then convert glucose-6-phosphate to free glucose. Figure\(\PageIndex{16}\) shows the unique phosphatase reaction.

Figure \(\PageIndex{16}\): Phosphorylase Enzymes utilized during Gluconeogenesis

First, let's consider the dephosphorylation of fructose 1,6-bisphosphate. The gluconeogenic enzyme is named fructose-1,6-bisphosphatase (FBP or FBPase-1). FBP requires a metal cofactor and is competitively and allosterically regulated. Fructose 2,6-bisphosphate and low-energy load (AMP and ADP) inhibit the enzyme. The overall reaction is shown below:

Fructose-1,6-bisphosphatase: F1,6BP + H₂O → F6P + P_i

Figure \(\PageIndex{17}\) shows the mechanism of mouse fructose,1-6-bisphosphatase (FBPase-1)

Figure \(\PageIndex{17}\): Mechanism of mouse fructose,1-6-bisphosphatase. Mg²⁺ ions coordinate the positioning of F-1,6-BP near an activated water molecule that mediates nucleophilic attack on the 1-position phosphate group. This leads to the cleavage of the phosphate group from the sugar molecule, F6P, which can then leave the active site.

Figure\(\PageIndex{18}\) below shows an interactive iCn3D model of pig fructose-1,6-bisphosphatase with bound Mg²⁺, fructose-6-phosphate, and phosphate in the R state (1EYI).

Figure \(\PageIndex{18}\): Fructose-1,6-bisphosphatase with bound Mg, fructose-6-phosphate and phosphate in the R state (1EYI). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...DbWMHBRTdPGhJ8

FBP is an allosteric homotetramer and can exist in a tense (T) and relaxed (R) state. The enzyme exhibits the highest catalytic activity in the R state. Figure\(\PageIndex{18}\) shows only one monomer of the R state with bound ligands and substrates for clarity.

Figure \(\PageIndex{19}\) shows an interactive iCn3D model of tetrameric FBP enzyme in the T state with bound Mg²⁺, fructose-6-phosphate, phosphate, and AMP, an allosteric inhibitor, in the T state (1EYJ).

Figure \(\PageIndex{19}\): Tetrameric fructose-1,6-bisphosphatase in the T state with bound Mg²⁺, fructose-6-phosphate, phosphate and AMP in the T state (1EYJ). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...UAbjNtkPsoBdA9

FBPase is inhibited by AMP and fructose-2,6-bisphosphate and activated by ATP.  Allosteric changes control the enzyme's catalytic activity and the transition between the T and R states. Figure\(\PageIndex{17}\) shows that FBP has three metal binding sites. The mechanism is shown in Figure \(\PageIndex{17}\). All binding sites in the model contain Mg2⁺; however, Zn²⁺ can also bind. In the presence of AMP, only one site is occupied by Mg²⁺, as shown in the model of the T state in Figure\(\PageIndex{19}\). The T-state form of FBP has a disordered loop (52-72). In the R state (no AMP), the loop interacts with the active site, and 1 Zn²⁺ and 2 Mg²⁺ ions are bound in the three sites. The binding of AMP leads to the dissociation of two of the bound metal cofactors and causes a decrease in enzyme activity.

The resulting fructose-6-phosphate is isomerized to glucose-6-phosphate. Glucose 6-phosphate is then converted by the last unique gluconeogenic enzyme, glucose 6-phosphatase, to generate free glucose. The overall reaction is shown below:

Glucose-6-phosphatase: G6P + H₂O → glucose + P_i

The dephosphorylation of glucose occurs only appreciably in liver cells, as this is the primary site for regulating blood glucose levels. This serves as the final step in the gluconeogenic pathway. The glucose-6-phosphatase enzyme is a transmembrane protein that resides in the endoplasmic reticulum inner membrane. Thus, for glucose 6-phosphate to be dephosphorylated, it must first be transported from the cytoplasm into the lumen of the endoplasmic reticulum (ER) through transporter 1 (T1) (Figure\(\PageIndex{20}\)). The glucose 6-phosphatase (G-6-Pase) then cleaves the phosphate from the substrate, releasing inorganic phosphate (P) and glucose (red molecule). Inorganic phosphate is then transported back into the cytoplasm via transporter 2 (T2), and glucose is transported via transporter 3 (T3). Free glucose is then transported back into the bloodstream through a glucose (GLUT) transporter (not shown in Figure\(\PageIndex{20}\)).

Figure\(\PageIndex{20}\) Dephosphorylation of Glucose 6-Phosphate in the Lumen of the Endoplasmic Reticulum.

The catalytic site of this hydrolase enzyme uses Lys76, Arg83, His119, Arg170, and His176. His 176 acts as a nucleophile that attacks the phosphorus of G6P in an S_N2-like reaction to form a His-PO₃^2- intermediate. Hydrolysis follows, restoring the enzyme to its original state. Figure \(\PageIndex{21}\) shows an abbreviated mechanism for the reaction.

Figure \(\PageIndex{21}\): Abbreviate mechanism for human glucose-6-phosphatase

Figure \(\PageIndex{21}\) shows an interactive iCn3D model of the AlphaFold predicted structure of human glucose-6-phosphatase (P35575). The enzyme contains nine transmembrane helices that anchor the protein in the ER inner membrane. The active site faces the ER lumen.

Figure \(\PageIndex{14}\): AlphaFold predicted structure of human glucose-6-phosphatase (P35575)(Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...jmcQAo49eZ8p16

Summary:

(Summary written by Claude, Sonnet 4.6, Anthropic)

This chapter describes gluconeogenesis — the synthesis of glucose from non-carbohydrate precursors — as a pathway that is both complementary to and reciprocally regulated with glycolysis, ensuring that blood glucose levels are maintained during fasting, starvation, and intense exercise.

Gluconeogenesis occurs primarily in the liver and, to a lesser extent, the renal cortex. Its principal substrates are lactate (recycled from anaerobic muscle metabolism via the Cori cycle), glucogenic amino acids (whose carbon skeletons enter as pyruvate, oxaloacetate, or other TCA cycle intermediates after transamination or deamination), and glycerol (derived from triacylglycerol hydrolysis). The pathway largely reuses the reversible enzymes of glycolysis in the reverse direction, but three glycolytic steps are irreversible under physiological conditions and require energetically distinct bypass reactions: the pyruvate kinase reaction, the phosphofructokinase-1 reaction, and the hexokinase/glucokinase reaction.

The first bypass — converting pyruvate to phosphoenolpyruvate — requires two enzymes and consumes one ATP and one GTP. Pyruvate carboxylase, a biotin-dependent tetrameric enzyme in the mitochondrial matrix, first carboxylates pyruvate to oxaloacetate. The reaction proceeds in two spatially separated steps within the enzyme: bicarbonate is first activated by ATP and fixed onto the biotin cofactor in the biotin carboxylase (BC) domain, and the carboxybiotinyl arm then swings on its flexible BCCP tether to the carboxyl transferase (CT) domain, where the activated CO₂ is transferred to the enolate form of pyruvate to yield oxaloacetate. Acetyl-CoA is an obligate allosteric activator. Because oxaloacetate cannot be directly transported across the inner mitochondrial membrane, it must be converted to aspartate (or malate) by mitochondrial aspartate aminotransferase (or malate dehydrogenase), exported via specific antiporters as part of the malate-aspartate shuttle, and reconverted to oxaloacetate in the cytosol. PEPCK then catalyzes the GTP-dependent decarboxylation and phosphorylation of oxaloacetate to form PEP, completing the first bypass.

The second bypass replaces the PFK1 reaction. Fructose-1,6-bisphosphatase (FBPase-1) catalyzes the Mg²⁺-dependent hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and Pi. As an allosteric homotetramer with T and R states, FBPase-1 is inhibited by AMP and fructose-2,6-bisphosphate (the same signal that activates PFK1) and activated by ATP — a regulatory arrangement that ensures glycolysis and gluconeogenesis are not simultaneously active. The loss of two metal-ion cofactors upon AMP binding stabilizes the catalytically inactive T state.

The third bypass replaces the hexokinase reaction. Glucose-6-phosphatase, an ER-resident transmembrane enzyme with its active site facing the ER lumen, hydrolyzes glucose-6-phosphate to free glucose via a covalent histidine-phosphate intermediate. Because glucose-6-phosphatase is expressed almost exclusively in the liver and kidneys, it is these organs that can export free glucose into the bloodstream. The glucose-6-phosphate substrate must be transported into the ER lumen by a dedicated transporter before dephosphorylation can occur, and the products — Pi and glucose — are exported by separate transporters. This compartmentalization provides an additional level of control over glucose output from the liver, the primary organ responsible for maintaining blood glucose homeostasis.

References

Principles of Biochemistry/Gluconeogenesis and Glycogenesis. (2019, February 8). Wikibooks, The Free Textbook Project. Retrieved 22:01, August 11, 2022, from https://en.wikibooks.org/w/index.php?title=Principles_of_Biochemistry/Gluconeogenesis_and_Glycogenesis&oldid=3515124.