12 Citric Acid Cycle as Source of Biosynthetic Precursor

Dr. Padma Ambalam

epgp books

 

 

 

 

 Citric Acid Cycle as source of Biosynthetic Precursor

 

Objectives

1. To understand the function of a central metabolic pathway that produces precursors and substrates used in biosynthetic processes is the TCA cycle.

2. To examine the importance of citric acid cycle intermediates in the synthesis of amino acids, carbohydrates etc

3. To relate defects inBiogenesis or anabolism, requires substrates to be acted upon that result in the formation of larger more complex molecules.

4. The citric acid cycle is a source of biosynthetic precursors & thus serves as metabolic integration hub.

 

 

 

Introduction

  • The citric acid cycle is involved in both catabolic and anabolic processes, it is known as an amphibole pathway.
  • It is the major degradative pathway for the generation of ATP and its intermediates must be replenished if any are drawn off for biosynthesis.
  • Several intermediates of TCA cycle are used for the synthesis of important compounds, which will have significant cataplerotic effects on the cycle.
  • Acetyl-CoA cannot be transported out of the mitochondria. To obtain cytosolic acetyl-CoA, citrate is removed from the citric acid cycle and carried across the inner mitochondrial membrane into the cytosol.
  • The oxaloacetate can be used for gluconeogenesis (in the liver), or it can be returned into mitochondria as malate.
  • The carbon skeletons of many non-essential amino acids are made from citric acid cycle intermediates.
  • In this reaction the glutamate is converted into alpha-ketoglutarate, which is a citric acid cycle intermediate. The intermediates that can provide the carbon skeletons for amino acid synthesis are oxaloacetate which forms aspartate and asparagine; and alpha-ketoglutarate which forms glutamine, proline, and arginine (Figure-1).
  • Of these amino acids, aspartate and glutamine are used, together with carbon and nitrogen atoms from other sources, to form the purines that are used as the bases in DNA and RNA, as well as in ATP, AMP, GTP, NAD, FAD and CoA.
  • The pyrimidines are partly assembled from aspartate (derived from oxaloacetate). The pyrimidines, thymine, cytosine and uracil, form the complementary bases to the purine bases in DNA and RNA, and are also components of CTP, UMP, UDP and UTP.
  • The majority of the carbon atoms in the porphyrins come from the citric acid cycle intermediate, Succinyl-CoA. These molecules are an important component of the hemoproteins, such as hemoglobin, myoglobin and various cytochromes.During gluconeogenesis mitochondrial oxaloacetate is reduced to malate which is then transported out of the mitochondria, to be oxidized back to oxaloacetate in the cytosol.

Figure-1  Overview of Citric Acid Cycle as a source of Biosynthetic Precursor.(Adapted from Garrett and Grisham: Biochemistry 2nd edition).

 

  • Oxaloacetate is converted to glucose in gluconeogenesis. Succinyl-CoA is a central intermediate in the synthesis of the porphyrins ring of Heme groups, which serve as oxygen carriers (in haemoglobin and myoglobin) and electronCarriers (in cytochromes). And the citrate produced in some organisms is used commercially for a variety of purposes.
  • A Transamination reaction converts glutamate into α -ketoglutarate or vice versa. Glutamate is a precursor for the synthesis of other amino acids and purine nucleotides (Figure-2).
  • Succinyl-CoA is a precursor for porphyrins. Oxaloacetate can be transaminated to form aspartate. Aspartate itself is a precursor for other amino acids and pyrimidine nucleotides. Oxaloacetate is a substrate for gluconeogenesis.

Figure-2 Role of citric acid cycle in anabolism. (Adapted from

http://www.up.edu.ps/ocw/repositories/pdf-archive/MGDC2202/body.html)

 

   Mechanism of synthesis of Leucine, Alanine and Valine from pyruvate

  • Pyruvate is the end result of glycolysis and can feed into both the TCA cycle and fermentation processes.Reactions beginning with either one or two molecules of pyruvate cause the synthesis of alanine, valine, and leucine. Feedback inhibition of final products is the main method of inhibition, and, in E. coli, the ilvEDA operon also plays a part in this regulation.
  • Valine is produced by a four-enzyme pathway. It begins with the reaction of two pyruvate molecules catalyzed by Acetohydroxy acid synthase yielding α-acetolactate. Step two is the NADPH+ + H+ – dependent reduction of α-acetolactate and migration of the methane groups to produce α, β-dihydroxyisovalerate.
  • This is catalyzed by Acetohydroxyisomer reductase. The third reaction is the dehydration reaction of α, β-dihydroxyisovalerate catalyzed by Dihydroxy acid dehydrase resulting in α-ketoisovalerate. Finally, a transamination catalyzed either by an alanine-valine transaminase or a glutamate-valine transaminase results in valine (Figure-4).
  • The leucine synthesis pathway diverges from the valine pathway beginning with α-ketoisovalerate. α-Isopropyl malate synthase reacts with this substrate and Acetyl CoA to produce α-isopropyl malate. An isomerase then isomerizes α-isopropyl malate to β-isopropyl malate.
  • The third step is the NAD+-dependent oxidation of β-isopropyl malate via the action of a dehydrogenase to yield α-ketoisocaproate. Finally is the transamination via the action of a glutamate-leucine transaminase to result in leucine (Figure-4).

Figure-3 the biosynthesis of Leu and Val from pyruvate. The action of Aceto hydroxy acid synthase (AHAS), ketoacid reductoisomerase (KARI), and dihydroxy acid dehydratase (DHAD) yield 2-oxo isovalerate that is either transaminated to Valor subjected to additional reactions specific for Leu biosynthesis.(Adapted from Plant Physiology February 2007 vol. 143 no. 2 / 970-986 )

 

  • Alanine is produced by the transamination of one molecule of pyruvate using two alternate steps: 1) conversion of glutamate to α-ketoglutarate using a glutamate-alanine transaminase, and 2) conversion of valine to α-ketoisovalerate via Transaminase C. (Figure-4)
  • Not much is known about the regulation of alanine synthesis. The only definite method is the bacterium’s ability to repress Transaminase C activity by either valine or leucine (seeilvEDA operon). Other than that, alanine biosynthesis does not seen to be regulated.

Figure-4 conversion of Pyruvate to Alanine.(Adapted from

http://www.namrata.co/subjective-questions-fate-of-pyruvate/)

 

Mechanism of conversion of Acetyl CoA to fatty acids and steroids

  • Acetyl CoA itself is a major biosynthetic precursor for the formation of lipids.
  • Lipogenesis is the process by which acetyl-CoA is converted to fatty acids. Through lipogenesis and subsequent triglyceride synthesis, the energy can be efficiently stored in the form of fats.
  • Lipogenesis encompasses both the process of fatty acid synthesis and triglyceride synthesis (where fatty acids are esterified with glycerol to form fats). The  products  are  secreted  from the liver in the form of very-low-density lipoproteins (VLDL).
  • Fatty acids synthesis starts with acetyl-CoA and builds up by the addition of two-carbonunits. The synthesis occurs in the cytoplasm of the cell, in contrast to the degradation (oxidation), (Figure-5) which occurs in the mitochondria.
  • Many of the enzymes for the fatty acid synthesis are organized into a multienzyme complex called fatty acid synthetase. The major sites of fatty acid synthesis are adipose tissue and the liver.

 

Acetyl-CoA carboxylase

  • Insulin affects ACC in a similar way to PDH. It leads to its dephosphorylation which activates the enzyme. Glucagon has an antagonistic effect and increases phosphorylation, deactivation, thereby inhibiting ACC and slowing fat synthesis.(Figure-5)
  • Affecting ACC affects the rate of acetyl-CoA conversion to malonyl-CoA. Increased malonyl-CoA level pushes the equilibrium over to increase production of fatty acids through biosynthesis.

Figure-5 Conversion of Acetyl CoA to long chain fatty acids (Adapted from Agronomy 317 – Principles of Weed Science Authored by Dr. Lance R. Gibson Copyright © 2001 Iowa State University. All rights reserved. Revised: July 23, 2004.)

 

  • Steroid biosynthesis is an anabolic metabolic pathway that produces steroids from simple precursors. This pathway is carried out in different ways in animals than in many other organisms, making the pathway a common target for antibiotics and other anti-infective drugs.
  • In addition, steroid metabolism in humans is the target of cholesterol-lowering drugs such as statins. It starts in the mevalonate pathway in humans, with Acetyl -CoA as building blocks, which form DMAPP and IPP. In following steps, DMAPP and IPP form lanosterol, the first steroid.
  • Steroidogenesis is the biological process by which steroids are generated from cholesterol and transformed into other steroids. The pathways of steroidogenesis differ between different species, but the pathways of human steroidogenesis are shown in the figure. Products of steroidogenesis include: androgens testosterone estrogens and progesterone corticoids cortisol aldosterone

 

 

The process of cholesterol synthesis can be considered to be composed of five major steps: (Figure-6)

  1. Acetyl-CoAs are converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)
  2. HMG-CoA is converted to mevalonate
  3. Mevalonate is converted to the isoprene based molecule, isopentenylpyrophosphate (IPP), with the concomitant loss of CO2
  4. IPP is converted to squalene
  5. Squalene is converted to cholesterol.

 

Figure -6 Pathway of cholesterol biosynthesis. Synthesis of cholesterol begins with the transport of acetyl-CoA from within the mitochondria to the cytosol. The rate limiting step in cholesterol synthesis occurs at the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, HMGR, cata+lyzed step. The phosphorylation reactions are required to solubilize the isoprenoid intermediates in the pathway. Intermediates in the pathway are used for the synthesis of prenylated proteins, dolichol, coenzyme Q and the side chain of Heme a. The abbreviation “PP” (e.g. isopentenyl-PP) stands for pyrophosphate. ACAT2: acetyl-CoA acetyltransferase. HMGCS1: HG-CoA synthase 1 (cytosolic). HMGCR: HMG-CoA reductase. MVK: mevalonate kinase. PMVK: phosphomevalonate kinase. MVD: diphosphomevalonate decarboxylase. IDI1/IDI2: isopentenyl-diphosphate delta isomerase 1 and 2. FDPS: farnesyl diphosphate synthase. GGPS1: geranylgeranyl diphosphate synthase 1. FDFT1: farnesyl-diphosphate farnesyltransferase 1 (more commonly called squalene synthase). SQLE: squalene-epoxidase (also called squalene monooxygenase). LSS: lanosterol synthase (2, 3-oxidosqualene-lanosterol cyclase). DHCR7: 7-dehydrocholesterol reductase. (Adaptedfrom © 1996–2014 themedicalbiochemistrypage.org, LLC | info @ themedicalbiochemistrypage.org)

 

 

Mechanism of conversion of α-ketoglutarate to glutamate

  • The α-ketoglutarate to glutamate reaction does not occur in mammals, as glutamate dehydrogenase equilibrium favours the production of ammonia and α-ketoglutarate.
  • Glutamate dehydrogenase also has a very low affinity for ammonia (high Michaelis  constant  of about 1 mM), and therefore toxic levels of ammonia would have to be present in the body for the reverse reaction to proceed (that is, α-ketoglutarate and ammonia to glutamate and NAD (P) +).
  • In bacteria, the ammonia is assimilated to amino acids via glutamate and aminotransferases.
  • In plants, the enzyme can work in either direction depending on environment and stress. Transgenic plants expressing microbial GLDHs are improved in tolerance to herbicide, water deficit, and pathogen infections. They are more nutritionally valuable.
  • Glutamate can by synthesized by the addition of ammonia to a-ketoglutarate
  • Aspartate is synthesize by the transfer of an ammonia group from glutamate to oxaloacetate.
  • Alanine synthesis is a bit of a mystery. Several reactions have been identified, but it has been impossible to generate an alanineauxotroph and therefore positively identify a required pathway. There are several pathways and the most likely is formation of  alanine by transamination from glutamate onto pyruvate. A transamination using valine instead of glutamate is also possible
  • Proline is synthesized by dehydration of Glutamate.

 

Mechanism of conversion of Succinyl CoA to porphyrins

  • WhereSuccinyl-CoA and glycine are combined by ALA synthase to form δ-aminolevulinic acid (dALA).
  • Porphyrins are complex chemical compounds that are large heterocyclic organic ring structures. The complex ring structures of porphyrins are composed of four modified pyrrole (5-membered organic ring) subunits connected by methine (=CH-) bridges.
  • The naturally occurring porphyrins of biological significance are the hemes. Hemes in biological systems consist of ferrous iron (Fe2+) complexed with four nitrogens of the specific porphyrin molecule identified as protoporphyrin IX. (Figure- 7)

Figure – 7 conversion of Succinyl CoA to porphyrins. (Adapted fromhttp://www.llamp((Liverpool) Library of Apicomplexan Metabolic Pathways) .net/? q=Nca%20Porphyrin%20metabolism)

   

Mechanism of conversion of fumarate and oxaloacetate to Aspartate

  • The enzyme aspartokinase, which catalyzes the phosphorylation of aspartate and initiates its conversion into other amino acids, can be broken up into 3 isozymes, AK-I, II and III. (Figure-8)

Figure – 8 conversions of fumarate and oxaloacetate to Aspartate.(Adapted from  http://krebbing.blogspot.in/2006_12_01_archive.htmland

http://chemistry.tutorvista.com/biochemistry/asparagine.html

 

Mechanism of conversion of oxaloacetate to Lysine

  • Lysine is synthesized from aspartate via the diaminopimelate (DAP) pathway. The initial two stages of the DAP pathway are catalyzed by aspartokinase and aspartate semi aldehyde dehydrogenase and play a key role in the biosynthesis of lysine, threonine and methionine(Figure-9).
  • There are two functional aspartokinase/homoserine dehydrogenases, ThrA and MetL, in addition to a monofunctional aspartokinase, LysC.

Figure – 9conversion of oxaloacetate to Lysine. (Adapted from

http://lecturer.ukdw.ac.id/dhira/Metabolism/aminoacids.html ©2000 Timothy Paustian,

University of Wisconsin-Madison)

 

Mechanism of conversion of oxaloacetate to Carbohydrates

 

Gluconeogenesis is a pathway consisting of a series of eleven enzyme-catalyzed reactions. The pathway may begin in the mitochondria or cytoplasm (of the liver/kidney), this being dependent on the substrate being used. Many of the reactions are the reversible steps found in glycolysis. (Figure-10)

 

  • Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate by the carboxylation of pyruvate.
  • Oxaloacetate is reduced to malate using NADH, a step required for its transportation out of the mitochondria.
  • Malate is oxidized to oxaloacetate using NAD+ in the cytosol, where the remaining steps of gluconeogenesis take place.
  • Oxaloacetate is decarboxylated and then phosphorylated to from phosphoenolpyruvate using the enzyme PEPCK. A molecule of GTP is hydrolyzed to GDP during this reaction.
  • The next steps in the reaction are the same as reversed glycolysis. However, fructose 1, 6-bisphosphatase converts fructose to fructose 6-phosphate, using one water molecule and releasing one phosphate (in glycolysis phosphofructokinase 1 converts F6P to F1, 6BP). This is also the rate-limiting step of gluconeogenesis.
  • Glucose-6-phosphate is formed from fructose 6 phosphate by phosphoglucoisomerase (the reverse of step 2 in glycolysis). Glucose-6-phosphate can be used in other metabolic pathways or dephosphorylated to free glucose. Whereas free glucose can easily diffuse in and out of the cell, the phosphorylated form (glucose-6-phosphate) is locked in the cell, a mechanism by which intracellular glucose levels are controlled by cells.
  • The final reaction of gluconeogenesis, the formation of glucose, occurs in the lumen of the endoplasmic reticulum, where glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase to produce glucose and release an inorganic phosphate. Like two steps prior, this step is not a simple reversal of glycolysis, in which hexokinase catalyzes the conversion of glucose and ATP into G6P and ADP. Glucose is shuttled into the cytoplasm by glucose transporters located in the endoplasmic reticulum’s membrane.

Figure – 10 conversion of oxaloacetate to Carbohydrates. (Adapted from

http://www.namrata.co/gluconeogenesis-subjective-questions-solved-part-1/)

   Mechanism of conversion of oxaloacetate to Isoleucine

 

The enzymes threonine deaminase, Dihydroxy acid dehydrase and transaminase are controlled by end-product regulation. (Figure-11)

 

Figure – 11conversion of oxaloacetate to Isoleucine. (Adapted from http://lecturer.ukdw.ac.id/dhira/Metabolism/aminoacids.html ©2000 Timothy Paustian, University of Wisconsin-Madison)

 

SUMMARY

  • The TCA cycle is the final common pathway for oxidation of fuel molecules which oxidizes C2 units. It takes place in the mitochondrial matrix.
  • The TCA cycle is regulated according to energy charge via pyruvate dehydrogenase, isocitrate dehydrogenaseandα-ketoglutarate dehydrogenase all of which are inhibited by NADH and ATP.
  • It is most likely that the citric acid cycle was assembled from pre-existing reaction pathways. As noted earlier, many of the intermediates formed in the citric acid cycle are used in biosynthetic pathways to generate amino acids and porphyrins.
  • Thus, compounds such as pyruvate, α-ketoglutarate, and oxaloacetate were likely present early in evolution for biosynthetic purposes. The oxidative decarboxylation of these α-ketoacids is quite favourable thermodynamically.
  • The elegant modular structures of the pyruvate and α-ketoglutarate dehydrogenase complexes reveal how three reactions (decarboxylation, oxidation, and thioester formation) can be linked to harness the free energy associated with decarboxylation to drive the synthesis of both acyl CoA derivatives and NADH.

 

 

Intermediates of citric acid cycle are also precursors of:

  • Glucose biosynthesis.
  • Lipid biosynthesis including fatty acid and cholesterol.

 

Note: Lipid biosynthesis is taken place in cytosol, but the mitochondrial acetyl -CoA (processor) cannot be transported across the inner mitochondrial membrane. Thus, acetyl- CoA is converted to citrate by ATP-citrate lyase since citrate can cross the membrane.

 

Why citrate synthase is not used? — Because no ATP is produced. ADP + Pi + oxaloacetate + acetyl-CoA ↔ATP + citrate + CoA

 

  • Amino acid biosynthesis

α-ketoglutarate + NAD (P)H + NH4 +↔Glu + NAD(P)+ + H2O α-ketoglutarate + Ala ↔ Glu + pyruvate

 

Oxaloacetate + Ala ↔Asp + pyruvate

 

  • Porphyrin biosynthesis

UtilizesSuccinyl-CoA as a starting material.

 

  • When the citric acid cycle intermediates are transported too much as precursors, the concentration of oxaloacetate is very low. In this case, it is necessary to replenish citric acid cycle intermediates. The main reaction is:

 

Pyruvate + CO2 + ATP + H2O ↔ oxaloacetate + ADP + Pi

 

The citric acid cycle is truly at the centre of metabolism

 

Reduced products: NADH and FADH2 are reoxidized to produce ATP.

 

The citric acid intermediates are utilized in the biosynthesis of many vital cellular constituents.

 

Figure – 12metabolic flux in aerobic conditions. In the diagram, only the formation of (G)ATP is indicated (not its consumption).(Adapted from worm book the online review of C. elegans biology. Bart P. Braeckman, Koen Houthoofd, Jacques R. Vanfleteren§Biology Department, Ghent University, K.L.Ledeganckstraat 35, B-9000 Ghent, Belgium.This diagram is primarily based on the work of McElwee et al. (2006), Holt and Riddle (2003) and Burnell et al., (2005)).

   

 Regulation of citric acid cycle as a source of biosynthetic Precursor

  • While most steps in gluconeogenesis are the reverse of those found in glycolysis, three regulated and strongly endergonic reactions are replaced with more kinetically favorable reactions. Hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase enzymes of glycolysis are replaced with glucose-6-phosphatase, fructose-1,6-bisphosphatase, andPEP carboxykinase. This system of reciprocal control allow glycolysis and gluconeogenesis to inhibit each other and prevent the formation of a futile cycle.
  • Most factors that regulate the activity of the gluconeogenesis pathway do so by inhibiting the activity or expression of key enzymes. However, both acetyl CoA and citrate activate gluconeogenesis enzymes (pyruvate carboxylase and fructose-1, 6-bisphosphatase, respectively).
  • Global control of gluconeogenesis is mediated by glucagon (released when blood glucose is low); it triggers phosphorylation of enzymes and regulatory proteins by Protein Kinase A (a cyclic AMP regulated kinase) resulting in inhibition of glycolysis and stimulation of gluconeogenesis. Compensatory induction of gluconeogenesis occurs in the kidneys and intestine, driven by glucagon, glucocorticoids, and acidosis.

 

Regulation of conversion of pyruvate to Leucine

  • Leucine, like valine, regulates the first step of its pathway by inhibiting the action of the α-Isopropyl malate synthase. Because leucine is synthesized by a diversion from the valine synthetic pathway, the feedback inhibition of valine on its pathway also can inhibit the synthesis of leucine.

 

Regulation of conversion of Acetyl CoA to Fatty Acids

  • Insulin is a peptide hormone that is critical for managing the body’s metabolism. Insulin is released by the pancreas when blood sugar levels rise, and it has many effects that broadly promote the absorption and storage of sugars, including lipogenesis.
  • Insulin stimulates lipogenesis primarily by activating two enzymatic pathways. Pyruvate dehydrogenase (PDH), converts pyruvate into acetyl-CoA. Acetyl- CoA carboxylase (ACC), converts acetyl-CoA produced by PDH into malonyl-CoA. Malonyl-CoA provides the two-carbon building blocks that are used to create larger fatty acids.
  • Long chain fatty acids are negative allosteric regulators of ACC and so when the cell has sufficient long chain fatty acids, they will eventually inhibit ACC activity and stop fatty acid synthesis.
  • AMP and ATP concentrations of the cell act as a measure of the ATP needs of a cell. When ATP is depleted, there is a rise in 5’AMP. This rise activates AMP-activated protein kinase, which phosphorylates ACC and thereby inhibits fat synthesis.
  • This is a useful way to ensure that glucose is not diverted down a storage pathway in times when energy levels are low.
  • ACC is also activated by citrate. When there is abundant acetyl-CoA in the cell cytoplasm for fat synthesis, it proceeds at an appropriate rate.

 

Regulation of glutamate dehydrogenase for conversion of a- ketoglutarate to Glutamate

  • In humans, the activity of glutamate dehydrogenase is controlled through ADP-ribosylation, a covalent modification carried out by the gene sirt4. This regulation is relaxed in response to restriction and low blood glucose.
  • Under these circumstances, glutamate dehydrogenase activity is raised in order to increase the amount of α-ketoglutarate produced, which can be used to provide energy by being used in the citric acid cycle to ultimately produce ATP.
  • The control of GDH through ADP-ribosylation is particularly important in insulin-producing β cells. Beta cells secrete insulin in response to an increase in the ATP:
  • ADP ratio, and, as amino acids are broken down by GDH into α-ketoglutarate, this ratio rises and more insulin is secreted.
  • SIRT4 is necessary to regulate the metabolism of amino acids as a method of controlling insulin secretion and regulating blood glucose levels.
  • Mutations alter the allosteric binding site of GTP cause permanent activation of glutamate dehydrogenase lead to disorder known as hyperinsulinism-hyperammonemia.

 

Regulation of α-Ketoglutarate

  • The α-ketoglutarate family of amino acid synthesis (synthesis of glutamate, glutamine, proline and arginine) begins with α-ketoglutarate, an intermediate in the Citric Acid Cycle. The concentration of α-ketoglutarate is dependent on the activity and metabolism within the cell along with the regulation of enzymatic activity. This is one of the initial regulations of the α-ketoglutarate family of amino acid synthesis.
  • The regulation of the synthesis of glutamate from α-ketoglutarate is subject to regulatory control of the Citric Acid Cycle as well as mass action dependent on the concentrations of reactants involved due to the reversible nature of the transamination and glutamate dehydrogenase reactions.
  • The regulation of proline biosynthesis can be dependent on the initial controlling step through negative feedback inhibition.

 

Mechanism of conversion of fumarate and oxaloacetate to Aspartate

  • AK-I is feedback inhibited by threonine, while AK-II and III are inhibited by lysine. As a side note, AK-III catalyzes the phosphorylation of aspartic acid that is the commitment step in this biosynthetic pathway. The higher the concentration of threonine or lysine, the more aspartate kinase becomes down-regulated.

 

Mechanism of conversion of oxaloacetate to Lysine

  • There are two bifunctional aspartokinase/homoserine dehydrogenases, ThrA and MetL, in addition to a monofunctional asparto kinase, LysC. Transcription of aspartokinase genes is regulated by concentrations of the subsequently produced amino acids, lysine, threonine and methionine.
  • The higher these amino acids concentrations, the less the gene is transcribed. ThrA and LysC are also feed-back inhibited by threonine and lysine. Finally, DAP decarboxylase LysA mediates the last step of the lysine synthesis and is common for all studied bacterial species.
  • The formation of aspartate kinase (AK), which catalyzes the phosphorylation of aspartate and initiates its conversion into other amino acids, is also inhibited by both lysine and threonine, which prevents the formation of the amino acids derived from aspartate.
  • Additionally, high lysine concentrations inhibit the activity of dihydrodipicolinate synthase (DHPS). So, in addition to inhibiting the first enzyme of the aspartate families’ biosynthetic pathway, lysine also inhibits the activity of the first enzyme after the branch point, i.e. the enzyme that is specific for lysine’s own synthesis.

 

 

Mechanism of conversion of oxaloacetate to Isoleucine

  • The presence of isoleucine will downregulate the formation of all three enzymes, resulting in the downregulation of threonine biosynthesis
  • High concentrations of isoleucine also result in the downregulation of aspartate’s conversion into the aspartyl-phosphate intermediate, hence halting further biosynthesis of lysine, methionine, threonine, and isoleucine.

 

you can view video on Citric Acid Cycle as Source of Biosynthetic Precursor

 

 

References

  • Kersten S (April  2001). “Mechanisms of nutritional and hormonal regulation of lipogenesis”. EMBO Rep.2 (4):6. 282– doi:10.1093/emboreports/kve071. PMC 1083868.PMID 11306547

  • Boundless. “Substrates for Biosynthesis.” Boundless Microbiology. Boundless, 15 Jul. 2015. Retrieved 18 Jul. 2015 from https://www.boundless.com/microbiology/textbooks/boundless-microbiology-textbook/microbial-metabolism-5/biosynthesis-52/substrates-for-biosynthesis-335-5473.

  • Krebs HA, Weitzman PDJ (1987). Krebs’ citric acid cycle: half a century and still turning. London: Biochemical Society. p. 25. ISBN 0-904498-22-0.

 

Web site

  • http://lecturer.ukdw.ac.id/dhira/Metabolism/aminoacids.html
  • https://en.wikipedia.org/wiki/Amino_acid_synthesis
  • https://www.tamu.edu/faculty/bmiles/lectures/regulationtca.pdf
  • www.ncbi.nlm.nih.gov › NCBI › Literature › Bookshelf

 

 

Books

  • Harper’sIllustrated Biochemistry by Robert K.. Murray, Daryl K. Granner, Peter A. Mayes, 26th Edition (2003)
  • Lehninger’S Principle of Biochemistry David L. Nelson and Michael M. Cox,5th Edition (2008)
  • Instant Notes: Biochemistry, 2nd Edition, B.D. Hames & N. M. Hooper (2005)