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Control of catabolic pathways

Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze non-reversible reactions. In other words, if the initial reaction takes place, the pathway is committed to proceeding with the remaining reactions. Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).

Glycolysis

The control of glycolysis begins with the first enzyme in the pathway, hexokinase ( [link] ). This enzyme catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The presence of the negatively charged phosphate in the molecule also prevents the sugar from leaving the cell. When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates when a later enzyme, phosphofructokinase, is inhibited.

This illustration shows that glycolysis is regulated via three key enzymes: hexokinase phosphofructokinase, and phosphoglycerate kinase. The first two enzymes hydrolyze an ATP and the third one produces ATP.
The glycolysis pathway is primarily regulated at the three key enzymatic steps (1, 2, and 7) as indicated. Note that the first two steps that are regulated occur early in the pathway and involve hydrolysis of ATP.

Phosphofructokinase is the main enzyme controlled in glycolysis. High levels of ATP, citrate, or a lower, more acidic pH decrease the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle. Fermentation, with its production of organic acids like lactic acid, frequently accounts for the increased acidity in a cell; however, the products of fermentation do not typically accumulate in cells.

The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. (Recall that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis.) The regulation of pyruvate kinase involves phosphorylation by a kinase (pyruvate kinase kinase), resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect).

If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: A kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.

Citric acid cycle

The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH ( [link] ). These enzymes are isocitrate dehydrogenase and α -ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases. α -Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA—a subsequent intermediate in the cycle—causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative, as the increased levels of the α -ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.

Electron transport chain

Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases, and now, ATP begins to build up in the cell. This change is the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain.

Visit this site to see an animation of the electron transport chain and ATP synthesis.

For a summary of feedback controls in cellular respiration, see [link] .

Summary of Feedback Controls in Cellular Respiration
Pathway Enzyme affected Elevated levels of effector Effect on pathway activity
glycolysis hexokinase glucose-6-phosphate decrease
phosphofructokinase low-energy charge (ATP, AMP), fructose-6-phosphate via fructose-2,6-bisphosphate increase
high-energy charge (ATP, AMP), citrate, acidic pH decrease
pyruvate kinase fructose-1,6-bisphosphate increase
high-energy charge (ATP, AMP), alanine decrease
pyruvate to acetyl CoA conversion pyruvate dehydrogenase ADP, pyruvate increase
acetyl CoA, ATP, NADH decrease
citric acid cycle isocitrate dehydrogenase ADP increase
ATP, NADH decrease
α -ketoglutarate dehydrogenase Calcium ions, ADP increase
ATP, NADH, succinyl CoA decrease
electron transport chain ADP increase
ATP decrease

Section summary

Cellular respiration is controlled by a variety of means. The entry of glucose into a cell is controlled by the transport proteins that aid glucose passage through the cell membrane. Most of the control of the respiration processes is accomplished through the control of specific enzymes in the pathways. This is a type of negative feedback, turning the enzymes off. The enzymes respond most often to the levels of the available nucleosides ATP, ADP, AMP, NAD + , and FAD. Other intermediates of the pathway also affect certain enzymes in the systems.

Questions & Answers

Please what is ovulation
Adusei Reply
What is cell division?
Adusei
What is copulation?
Adusei
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Beatrice Reply
this is a specialized part of the cell eg Nucleus
David Reply
what are the organelles?
Faith Reply
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mana Reply
reproduction is the process by which living organisms give rise to young ones of their own kind
Miriam
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Wengelawit
the production of new forms of life over time as documented in the fossil record.
mana
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Marvin
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Kevin Reply
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Raih Reply
what are the organelles in cell that involves in protein sythenis
Rita Reply
what is a melanin?
Judith Reply
what is telophase
Elphas
melanin in that black color we posse in our skins
Marvin
Why do parasites take on a parasitic life?
Asadullah Reply
what is cell
Ousman Reply
A cell is the basic structure unit of an organ
Yaa
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Igwe
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Bashir
Organisms are living things that can function on their own
EZEA
A cell is a functional basic unit of an organisms.
Evelyn
Organisms is a entity which consists of one or more cells and are able to undergo all life processes
Evelyn
A respiration is the physical and chemical break down of complex food substance into absorbable or simple form.
Evelyn
What will to a cell if the nucleus is removed..?
Goodrick Reply
When a cell nucleus removed then the cell will not be able to function properly, it will not be able to grow .All the metabolic functioning of the cell will stop .Without nucleus the cell will lose it's control. It can not carry out cellular reproduction .
Evelyn
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walid
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Rhoda Reply
Kidney play a very large role in human osoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney as tubules, which is controlled by hormones such as antrdiuvetic hormone (ADH) , aldosterone and angiotensin.
Evelyn
kidney is capable of osmoregulation in mammal ,
Judith
simple definition for respiratory system
Gift Reply
Respiratory system is a network of organs and tissues that helps you to breathe or help in getting the rids of oxygen and discharge of carbon dioxide in the body.
Ruhiyatu
What are uses of respiratory system
Joster
how it functions
Ousman
what is inresparetion
Fikkabex
Why do parasites take on a parasitic life?
Asadullah
A respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plant
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diseases of respiration
walid
when air enters to the body called inresparetion
walid
explain why plants responds to stimuli slowly than animals
Bekoe
how is a aerenchyma tissue adapted to its function
fatuma Reply
Have large air spaces that store air for gaseous exchange... Large air spaces also facilitate bouyancy.
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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