Regulation of Glycolysis and TCA cycle

Regulation of Glycolysis &

TCA cycle

Hello!I am Sameer Turki

This presentation has been split into two parts for better understanding.

1.Regulation of Glycolysis

REGULATION OF GLYCOLYSIS

Flux through a metabolic pathway can be regulated in several ways: 1. Availability of substrate2. Concentration of enzymes responsible for rate-

limiting steps 3. Allosteric regulation of enzymes 4. Covalent modification of enzymes

Three reactions of the glycolysis are the regulatory steps.1) Conversion of glucose to glucose-6-phosphate catalyzed by hexokinase.2) Fructose to fructose1, 6-bisphosphate catalyzed by phosphofructokinase3) Formation of pyruvate from PEP catalyzed by pyruvate kinase.

Of the 10 steps in the glycolytic pathway, three involve large negative ∆G and are essentially irreversible. These are steps 1 (phosphorylation of glucose), 3 (phosphorylation of fructose-6-phosphate) and 10 (transfer of phosphate from phosphoenolpyruvate to ADP). Net ∆G for glycolysis is about -23 kcal/mol.


In practice, we generally consider reactions where ∆G is larger than about -2 kcal/mol to be “irreversible”.


The concentration of these three enzymes in the cell is regulated by hormones that affect their rates of transcription. Insulin is a peptide hormone secreted by pancreatic β-cells in response to sudden increases in blood glucose levels. The general effect of insulin is to promote the storage of energy when food is available in abundance.

Glucagon is a different peptide hormone secreted by the pancreatic α-cells. Its secretion is stimulated by low blood glucose levels, and its general effect is to oppose the action of insulin.

Insulin upregulates the transcription of glucokinase, phosphofructokinase, and pyruvate kinase, while glucagon downregulates their transcription.


HexokinaseHexokinase performs step 1 of glycolysis in most tissues, including muscle and brain. It has a low Km (high affinity) for glucose, so it permits initiation of glycolysis even when blood glucose levels are relatively low. However, its Vmax is relatively low. Hexokinase is inhibited by the product of its reaction, glucose-6-phosphate. This is a very important regulatory step, since it prevents the consumption of too much cellular ATP to form G6P when glucose is not limiting.


Phosphofructokinase PFK catalyzes the rate-limiting step in glycolysis and is the

most important control point. It is also the first irreversible step that is unique to the glycolytic pathway; G6P can be used as an intermediate in other pathways including glycogen synthesis and the pentose phosphate pathway.

PFK is allosterically inhibited by ATP, so glycolysis is slowed when cellular ATP concentrations are high. ATP binds to a site on PFK distinct from the active site, causing a conformational change resulting in rotation of the positions of Arg162 and Glu161. In the high-affinity state, the positive charge on Arg162 stabilizes the negative charge on the phosphate of F6P, and Km is low. In the low-affinity state, the negative charge on Glu161 repels F6P.


The conformational transition between these two states is also regulated by cellular pH. Excess H+ ions favor the low affinity state. Thus when cellular lactate is high (usually when oxidative phosphorylation is inhibited), the rate of glycolysis is reduced, preventing further accumulation of intracellular acid. This regulation helps to minimize the risk of lactic acidosis when oxygen is scarce.


When cellular energy is limited, glycolysis should be upregulated. PFK is allosterically activated by high levels of AMP. AMP overcomes the inhibitory effect of ATP.

Another allosteric activator of PFK is fructose 2, 6 bisphosphate. F-2,6-BP is not an intermediate in the glycolytic pathway. F-2,6-BP also overcomes the inhibitory effect of ATP. F-2,6-BP is made from F6P by a specific kinase, phosphofructokinase 2 (PFK2). F-2,6-BP is also an important regulator of the process of gluconeogenesis, where glucose is synthesized from pyruvate.


Pyruvate kinase Pyruvate kinase is the third regulated enzyme of glycolysis.

Like PFK, pyruvate kinase is regulated both by allosteric effectors and by covalent modification (phosphorylation). Pyruvate kinase is activated by F-1,6-BP in the liver, a second example of feedforward stimulation. ATP and alanine (a biosynthetic product of pyruvate) act as allosteric inhibitors of pyruvate kinase.

Phosphorylation of pyruvate kinase is regulated by blood glucose level, just like PFK. High glucagon (low blood sugar) causes phosphorylation, which in this case renders the enzyme inactive.




Regulation of TCA cycle

2.

Factors that regulate TCA cycle:

i) Substrate availability ii) Product accumulation iii) Ratio of NADH/NAD+ and ATP/ADP

One of the important roles of the cycle is to provide reduced cofactors, such as NADH and FADH2.

These reduced cofactors are further oxidized by the electron transport chain which is localized in the inner mitochondrial membrane. Energy of the oxidation is conserved in the form of ATP.

So, the two most important molecules, which regulate the TCA cycle, are the ratio of NADH/NAD+ and ATP/ADP. If a cell is actively metabolizing, which means consuming ATP, ratio of ATP/ADP will be low, and the ratio of NADH/NAD+ will also be low.

REGULATION OF TCA CYCLE

It gives a signal to the cell to produce more NADH and ATP to meet the demand, so the TCA cycle will operate more efficiently, while in a resting cell, ATP and NADH will accumulate resulting in high ratios of ATP/ADP and NADH/NAD+. This will result in inhibition of the activity of the enzymes responsible for producing them.


Regulation points of TCA cycle

1. The cycle is regulated at the entry level of acetyl-CoA. Citrate synthase catalyzes the condensation reaction of acetyl-CoA with OAA. Availability of these substrates will regulate the activity of citrate synthase, which varies with the metabolic status of the cell. Accumulation of citrate, succinyl-CoA and ATP inhibits the activity of citrate synthase. This inhibition of citrate synthase by ATP is relieved by ADP accumulation.

2. Second point of TCA regulation is the reaction catalyzed by isocitrate dehydrogenase. NADH and ATP accumulation inhibits the activity of the enzyme

.


3. The third point of regulation of TCA cycle is the reaction catalyzedby the enzyme α–ketoglutaratedehydrogenase. Activity of this enzyme is inhibited by the products of the reaction i.e., succinyl-CoA, and NADH. When the products of the reaction accumulate the activity of enzyme is inhibited.


REFERENCES:

• Stanford Medicinehttp://imed.stanford.edu/curriculum/session6/content/09-Regulation_of_glycolysis.pdf

• ResearchGatehttps://www.researchgate.net/file.PostFileLoader.html?id=577bfd1d40485412a95d1903&assetKey=AS%3A380558044614656%401467743516991

• Virtual Learning Environmenthttp://vle.du.ac.in/mod/resource/view.php?id=12106

• Nelson, Cox, (2004), Lehninger Principles of Biochemistry,4th Edition

http://imed.stanford.edu/curriculum/session6/content/09-Regulation_of_glycolysis.pdf

https://www.researchgate.net/file.PostFileLoader.html?id=577bfd1d40485412a95d1903&assetKey=AS:380558044614656@1467743516991

http://vle.du.ac.in/mod/resource/view.php?id=12106

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Regulation of Glycolysis and TCA cycle