Subsequently, the sugars enter the pathways of carbohydrate metabolism in the liver cells or in the cells of our body tissues. An overview of carbohydrate metabolism (Fig. 5.1) shows the connections of glucose to almost all major groups of biomolecules in the cell.
Conversion of Glyceraldehyde 3-P to 3-Phosphoglycerate
Details of the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase to demonstrate substrate phosphorylation using inorganic phosphate. The reaction is highly exergonic and the stored energy from the previous reaction is used to generate ATP.
Conversion of 3-Phosphoglycerate to Pyruvate
Regulation of Hexokinase/Glucokinase
It is not inhibited by Glc 6-P, but it responds to the nutritional and hormonal state of the body. The regulator protein in the nucleus allows release of the enzyme from the nucleus into the cytosol for catalytic activity only when the concentration of glucose is high.
Regulation of Phosphofructokinase-1
Schematic representation of the bifunctional polypeptide having phosphofructokinase-2 and fructose 1,6-bisphosphatase-2 activities (PFK-2 and FBPase respectively). Schematic diagram of the effect of dietary condition on the release of glucagon and insulin and the resulting effect on the rate of glycolysis.
Regulation of Pyruvate Kinase
Glycolysis breaks down glucose to pyruvate in all tissues, while gluconeogenesis synthesizes glucose from pyruvate in the liver. Pyruvate is also linked to lipid metabolism via acetyl CoA and NADPH.H+, necessary precursors in the synthesis of fatty acids.
GLYCOLYSIS
In the aerobic state, pyruvate is oxidized to acetyl-CoA in the mitochondrion and enters the citric acid cycle with the ultimate goal of producing ATP and biosynthetic intermediates. In the next section, we will follow the aerobic metabolism of pyruvate, a pathway that has the potential to save energy as ATP.
Section 2
End-product inhibition of PDH
The inhibition is enhanced if acetyl-CoA is simultaneously produced by β-oxidation of fatty acids. Conversely, when the availability of C-2 unit for the TCA cycle is insufficient, the low concentrations of acetyl-CoA and NADH.H+ are ineffective and PDH actively oxidizes more pyruvate to acetyl-CoA.
Covalent modification of PDH
Muscle contraction is associated with intracellular release of Ca2+, which also acts via covalent modification to activate E1 so that the availability of acetyl-CoA for the TCA cycle is increased and more ATP can be generated. Insulin, secreted when blood levels of glucose are high, stimulates PDH phosphatase to dephosphorylate and activate E1 for increased oxidation of pyruvate.
OXIDATION OF PYRUVATE
Section 3
The rate of formation of α-ketoglutarate (α-KGA) is important in determining the overall rate of the TCA cycle. Acetyl-CoA is required in stoichiometric amounts for each turn of the cycle and is obtained from:. Catabolic role of the TCA cycle as shown in the metabolism of the carbon skeleton of isoleucine.
The rate of the TCA cycle is primarily directed at the energy needs of the cell at any particular time. All three regulatory enzymes of the TCA cycle are inhibited in this way by mass action. The rate of the TCA cycle is closely linked to glycolysis and the electron transport chain.
The subsequent reduction in NAD+ supply decreases the rate of the TCA cycle.
CITRIC ACID CYCLE
Section 4
Reactions of the Oxidative phase
Glucose enters the HMP pathway as glucose 6-phosphate (Glc 6-P), and its oxidative decarboxylation is carried out by a sequence of four reactions.
- Reactions of the non-oxidative phase
- Interconversion: (C 5 ↔ C 5 ↔ C 5 )
- Recombination 1: (C 5 + C 5 ↔ C 3 + C 7 )
- Recombination 2: (C 7 + C 3 ↔ C 4 + C 6 )
- Recombination 3: (C 5 +C 4 ↔ C 3 + C 6 )
- Requirement for NADPH and ribose 5-P is balanced
- NADPH required but ribose 5-P is not needed
The reactions of the non-oxidative phase follow the oxidative phase in non-dividing tissues that need more. We divide the non-oxidative sequence into 4 parts to allow a clear understanding of the reactions. The reactions of the HMP, glycolysis and gluconeogenesis take place in the cytosol and share common intermediates such as glyceraldehyde 3-P.
The fate of the triose phosphates is determined by the cell's need for pentose phosphates, NADPH and ATP. The first reaction of the oxidative phase, catalyzed by glucose-6-P-dehydrogenase, is irreversible and rate-limiting for the entire pathway. The rate of the non-oxidative branch of the pathway depends on the availability of substrates.
Most of the glucose 6-P is processed by glycolysis to give fructose 6-P and glyceraldehyde 3-P, after which the reverse reactions of transaldolase and transketolase convert these intermediates to ribose 5-P.
HEXOSE MONOPHOSPHATE PATHWAY
Section 5
Regulation of phosphorylase in the muscle
The action of one molecule of the hormone epinephrine (that is, the primary signal) ultimately leads to the activation of several molecules of the target enzyme, phosphorylase. Refer to Figure 5.5.9 as we trace the steps in the cascade mechanism of glycogen phosphorylase activation in muscle. Regulation of muscle glycogen phosphorylase by a cAMP-mediated enzyme cascade mechanism. Other factors in target enzyme regulation have also been demonstrated.
Epinephrine binds to β-receptors on the cell membrane of myocytes to trigger a cAMP- mediated cascade pathway for activating phosphorylase
The binding of epinephrine activates a membrane-bound enzyme adenylyl cyclase, which converts ATP to cyclic 3’,5’-AMP
The mechanism of protein kinase activation by cyclic AMP: On the left is the inactive holoenzyme, R2C2, in which R and C are regulatory and catalytic subunits, respectively.
Active cAMP-dependent protein kinase (PKA) performs two functions
Activated phosphorylase kinase phosphorylates PL b to the active form, PL a, which then starts the process of glycogenolysis
Regulation of Phosphorylase in the Liver
Glucagon, secreted by the pancreas when blood glucose falls, can also initiate the cAMP-dependent enzyme cascade pathway to activate phosphorylase. The hormones bind to an α1-adrenergic receptor in the plasma membrane and activate a G protein that stimulates phospholipase C. The resulting increase in inositol 1,4,5-triphosphate (IP3) induces Ca2+ mobilization from ER stores to the cytosol and activates phosphorylase. kinase.
Glucose now binds to an allosteric site on PL a, causing a conformational change that results in dephosphorylation and inactivation of the enzyme by PP-1. Insulin, secreted by the pancreas when blood sugar is high, stimulates PP-1, which inactivates both phosphorylase kinase and PL a. After a meal, when blood sugar levels are high and insulin is present, cells in the liver and muscles take up glucose for to replenish their glycogen stores.
The reactions are essentially the same in liver and muscle, although their regulation differs in detail in the two organs.
Steps in the synthesis of Glycogen by enlarging a “glycogen primer”
Conversion of glucose to UDP-glucose
The rapid hydrolysis of pyrophosphate by pyrophosphatase maintains the overall balance in favor of UDPG formation, although the mutase and pyrophosphorylase reactions are reversible. UDP-glucose serves as a glucosyl residue donor in the next step of glycogenesis.
Attachment of UDPG to “glycogen primer”
Glycogen synthase proceeds with the stepwise addition of more glucose residues to all linear chains of the pre-existing "primer".
Branching of linear chains
De novo synthesis of glycogen
- Epinephrine/Glucagon stimulate glycogenolysis but simultaneously suppress glycogenesis
- Insulin stimulates glycogenesis but simultaneously suppresses glycogenolysis
De novo synthesis of glycogen: a tyrosine residue in the protein glycogenin (represented by the blue oval) attaches glucosyl residues carried by UDP. Glycogen synthase is catalytically active only as long as it is in contact with glycogenin; this limits the size of the final glycogen molecule. Chief among these is glycogen synthase kinase-3 (GSK3), which strongly inhibits glycogen synthase by phosphorylating three Ser residues near the carboxy-terminal end of the enzyme.
Association with the glycogen-targeting protein, GM, brings PP-1 into close proximity to glycogen synthase and aids in catalysis. Hereditary deficiency of any of the enzymes of glycogen metabolism can result in deposition of an abnormal type or amount of glycogen in tissues. RECIPROCAL REGULATION OF GLYCOGENESIS AND GLYCOGENOLISIS Synthesis and breakdown of glycogen are coordinated in such a way that when one is stimulated, the other is inhibited.
From the sub-sections above you should have already realized that the actual controlling factors for glycogen synthase and phosphorylase are almost the same.
GLYCOGEN METABOLISM
Section 6
The three irreversible reactions of glycolysis are bypassed to allow for gluconeogenesis in the following way:
Pyruvate à Phosphoenolpyruvate
However, if PEP-CK is in the cytosol (eg, in rats), the malate shuttle (see Section 1) is used to transport OAA across the mitochondrial membrane into the cytosol for subsequent conversion to phosphoenolpyruvate. Two pathways for the conversion of pyruvate to phophoenolpyruvate: the location of the enzyme PEP carboxykinase determines whether PEP is produced in the cytosol or in the mitochondrion. Malate crosses the inner mitochondrial membrane and is reoxidized to OAA by cytosolic MDH.
The malate shuttle thus generates cytosolic NADH required to reverse the glyceraldehyde-3-P-dehydrogenase reaction of glycolysis so that glucose can be synthesized. Transaminases also enable a transfer of OAA to the cytosol, this time in the form of aspartate. Although ∆G'o is 0.9 kJ/mol for reaction 1, the actual ∆G under cellular conditions is -25 kJ/mol, and this makes the reaction irreversible.
Fructose 1,6-bisphosphate à Fructose 6-phosphate
Glucose 6-phosphate à Glucose
- Lactate
- Alanine
- Glycerol
- Propionate
- Hormonal regulation
The tissue-specific distribution of LDH isozymes allows rapid conversion of lactate to pyruvate in liver cells. Citrate, which is the condensation product of acetyl-CoA with OAA, allosterically stimulates FBPase-1. ADP and AMP increase in concentration when the energy state of the cell is low and this is not favorable for gluconeogenesis.
At any given time, the preferred route depends on blood glucose levels and on the energy state of the liver cells. When the energy state of the cell is low, [AMP] is high and the synthesis of ATP is required. This allows PFK-1 to respond to even small changes in the energy state of the cell and thereby control glycolysis.
The concentration of Fru 2,6-BP in the cell depends on the activity of the bifunctional enzyme (PFK-2 + FBPase-2) previously encountered in the regulation of glycolysis.
GLUCONEOGENSIS
In a normal adult, blood glucose levels are maintained between 4.5-5.5 mmol/L in the post-absorptive state. The primary organ in maintaining stable levels of glucose in the blood is the liver. As blood glucose levels rise (as in hepatic portal vein after a meal), the liver switches its enzymatic machinery from production to absorption of glucose.
It helps that unlike extrahepatic tissues, the liver has the required glucose transporter for rapid absorption of glucose from the blood when levels rise. When blood glucose is low, the liver shifts its own metabolism from using glucose for energy to the oxidation of fatty acids. The adrenal catecholamines and glucocorticoids tend to raise blood glucose levels when there is an increased demand for glucose in muscle and other tissues.
High blood levels of glucose between meals and low readings in a "glucose tolerance" test are indicative of the disorder.
REGULATION OF BLOOD GLUCOSE
