Living things need energy used for movement, growth, biomolecular synthesis, and ion transport across cell membranes. The organism will use this energy efficiently for living processes. To produce energy, carbohydrates, lipids, amino acid metabolism pathways through e different will be broken down and produce several carrier molecules energy I, one of which is through the Krebs cycle.

Energy carrier compound i is classified into two, namely low-energy phosphate s-ADP, AMP, glucose-1 phosphate charge capture the energy of free and high energy phosphate (HEP) -kreatin phosphate, ATP, carbamoyl phosphate, GTP, fosfoenol pyruvate which brings energy high to give to biochemical reactions.

There are three main sources of HEP compounds in energy conservation, namely from the process of glycolysis, the citric acid cycle, and oxidative phosphorylation. NADH which is the result of the Krebs cycle that occurs in the mitochondria will be used in the reduction reaction to produce ATP which is an energy-carrying molecule through oxidative phosphorylation processes.

What is the Krebs Cycle?

What is the Krebs Cycle?

The citric acid cycle or the Krebs cycle is a series of chemical reactions in cells, namely in the mitochondria, which take place sequentially and repeatedly. The purpose of this is to convert pyruvic acid to CO2, H2O, and some energy.

This reaction is a series of reactions that carry acetyl residual catabolism, freeing hydrogen equivalent, which by oxidation causes the release and capture of ATP as a tissue energy requirement. Acetyl residues in the form of acetyl-CoA (CH3-CO-S-CoA, active acetate). This process is an oxidation process using oxygen or aerobes.

The function of the Krebs cycle

  • Produces most CO2
  • Other metabolism that produces CO2, for example, the pentose phosphate pathway or P3 (pentose phosphate pathway) or if it is hampered hexose monophosphate.
  • Source of reduced enzymes that drive respiration chains
  • Is a tool so that excess energy can be used for fat synthesis before TG formation for fat storage
  • Provides important precursors for sub-units needed in the synthesis of various molecules
  • Provides direct or indirect control mechanisms for other enzyme systems

Krebs Cycle Process in the Body

Krebs Cycle Process in the Body

Stage I

Acetyl Co-A is formed in the reaction between pyruvate acid and coenzyme-A. Besides that, fatty acids can also produce acetyl Co- A in the oxidation process. Reaction of the formation of acetyl Co-A using pyruvate dehydrogenase complex as a catalyst consisting of several types of enzymes.

The coenzymes involved in this reaction are thiamine pyrophosphate (TPP), NAD +, lipoic acid and Mg + ions as activators. This reaction is not reversible.

The citrate synthase catalyzes the condensation reaction between acetyl coenzyme-A and oxaloacetate to produce citrate. This reaction is an aldol condensation reaction between the metal group and acetyl coenzyme-A and the carbonyl group of oxaloacetate where hydrolyzed thioester bonds occur and the formation of free coenzyme-A compounds. This reaction is exergonic hydrolysis that produces energy and is the first driving reaction for the Krebs cycle.

Stage II

Is the formation of isocitrates from citrate through cas-aconitate, catalyzed reversibly by the enzyme aconitase. This enzyme catalyzes the reversible reaction of adding H2O to the cis-aconitate double bond in two directions, one to the formation of citrate and the other to the formation of isocyanates.

Stage III

Oxidation of isocitrate to α-ketoglutarate takes place through the formation of a compound between oxalosucinate which is bound to the enzyme isocitrate dehydrogenase and NAD acts as its coenzyme.

The first enzyme catalyzes the oxidation process of isocitrate to oxalosucinate and decarboxylation of oxalosucinate to α-ketoglutarate. The conversion of isocitrate to oxaloacetate can be inhibited by diphenylchlorarsine, whereas oxaloacetate decarboxylation is inhibited by pyrophosphate.

Stage IV

Is the oxidation of α-ketoglutarate to succinate through the formation of succinyl coenzyme-A, which is a reaction that is reversible and catalyzed by the enzyme complex α-ketoglutarate dehydrogenase. This reaction is catalyzed by the succinyl coenzyme-A synthetase enzyme which is typical for GDP. Furthermore, GTP formed from this reaction is used for ATP synthesis of ADP with the nucleoside diphosphate kinase enzyme.

Stage V

Succinate is oxidized to fumarate by the enzyme succinate dehydrogenase that binds to flavin adenine dinucleotide (FAD) as its coenzyme. This enzyme is strongly bound to the inner membrane of the mitochondrion. In this reaction, FAD acts as a hydrogen receiver.

Stage VI

It is a reversible reaction to the addition of one H2O molecule to the fumarate double bond, producing L-malate, by catalyzing the fumarase enzyme without coenzyme. This enzyme is stereospecific, acting only on the L-stereoisomer form of malate. In this reaction, fumarase catalyzes the process of adding H atomic tras and OH groups to fumarate double bonds.

Reaction VII (end)

The final step in the citric acid cycle is dehydrogenation of malic acid to form oxaloacetic acid. L-malate is oxidized to oxaloacetate by the enzyme L-malate dehydrogenase which binds to NAD. This reaction is endergonic but the rate of reaction goes smoothly to the right.

This is possible because the next reaction, the oxaloacetate condensation reaction with acetyl coenzyme- A, is an irreversible exergonic reaction. Thus these reactions take place continuously and repeatedly.

The Krebs Cycle as a Series of Actions for Oxidation of the Arc in the Body

The Krebs Cycle as a Series of Actions

Mitochondria have to get the name right as the “powerhouse” of cells because in organelles here lasts arrest of energy derived from the oxidation of respiratory, system da l am mitochondrial pair respiration with the process of the formation of the intermediate high-energy ATP.

A number of specific enzymes act as markers for compartments that are separated by the mucochondrial membrane. Mitochondria have external membranes. The external membrane of selective permeability and composed in the form of folds or Krista, as well as the matrix in the internal membrane.

External membranes can be removed by reaction with digitonin and are characterized by the presence of monoamine oxidase, acyl-CoA synthetase, glycerophosphate acyltransferase, and phospholipase. Adenylkinase and keratin kinase are found in the space between membranes. Cardiolipid phospholipids are concentrated in the internal merman.

All the beneficial energy released during the oxidation of fatty acids and amino acids and almost all the energy released from carbohydrate oxidation is present in the mitochondria as the reducing element equivalent (H- or electron). Mitochondria contain a series of catalysts known as respiration chains.

What collects is the equivalent of the reducing element and the reaction with oxygen to form water. Also found in the mitochondria is a series of machines to capture free energy released as high-energy phosphates. Mitochondria also contain a variety of enzyme systems that are basically responsible for producing the majority of the reducing element Valene, namely enzymes β-o oxidation and the citric acid cycle.

The citric acid cycle is the last general metabolism for oxidation of all major ingredients.

The respiration chain in the mitochondria consists of a number of redox carriers that travel from the NAD-specific dehydrogenase system, through which all substrates are related to the respiration chain because the redox potential is more positive (for example, fumarate / succinate) directly related to the flavoprotein dehydrogenase system, which in turn will be linked to the respiration chain because the redox potential is more positive (for example, fumarate / succinate). Associated with cytochrome enzymes in the respiratory chain.

It is clear that there is an additional carrier in the respiration chain that links flavoprotein to cytochrome b, members of the cytochrome chain that have the lowest redox potential. This substance called ubiquinone or Q (coenzyme Q) is present in the mitochondria in the form of oxidized quinones under aerobic conditions and in reduced quinone form in anaerobic conditions.

mitochondria in the form of oxidized

Q is a constituent of mitochondrial lipids: pleated lipids are mainly present in the form of phospholipites that are part of the mitochondria. In the chloroplast. All of these substances are characterized by chains of piliisoprenoid. In the mitochondria, Q is present in excessive amounts of sitoikimetrik far greater than other members of respiration, this is in accordance with the function of Q which works as a component of the respiration chain car that collects the equivalent elements of reducing flavoprotein complexes which are more fixed and deliver to cytochromes.

An additional component found in the respiration chain preparation is iron-sulfur protein ( FeS; iron nonhem). This element binds to flavoprotein (metalloplavoprotein) and cytochrome b. sulfur and za t iron are thought to play a role in the oxidoreduction mechanism between flavin and Q which involves changes in only one e ‘single with iron atoms undergoing oxidoreduction between Fe2 + and Fe3 +. The enzyme dehydrogenase analyzes the process of electron transfer from the substrate to the chain NAD.

There are some differences in carrying out this process α – ketopiruvat keteloglutara, having a complex dehydrogenase system involving lipoates and FAD before electrons are transferred to the respiration chain NAD.

Electron transfer from other dehydrogenase enzymes such as 3-hydroxy acyl-CoA, 3-hydroxybutyrate, proline, glutamate, malate and isocitrate dehydrogenase directly paired with NAD ‘in the respiration chain. NADH (reduced) in the respiratory chain subsequent oxidation by metaloflavoprotein- enzyme NADH dehydrogenase.

This enzyme contains FeS and FMN, is tightly bound to the chain of respiration and delivers the equivalent reducing element to Q. Q is also a collecting point in the respiration chain for reducing equivalent elements derived from other substrates that bind directly to the respiration chain through the enzyme flavoprotein dehydrogenase.

This substrate includes succinate, choline, glycerol 3-phosphate, sarcosin, methylated, and acyl – CoA. Moiety flavin of all these dehydrogenase enzymes is FAD. Electrons flow from Q, through cytochrome sequences that are seen deep into oxygen molecules.

Cytochromes are arranged in an order of increasing redox potential. The cytochrome aa3 terminal group (cytochrome oxidase) is responsible for the final combination of a number of reducing elements with oxygen molecules. This enzyme system turned out to contain copper, a component found in several oxidase enzymes.

Oxidation cannot take place through the respiration chain if at the same time there is no Krebs cycle . Chance and William mentioned 5 conditions that can control the rate of respiration in the mitochondria. Generally, most resting cells are in status 4 and respiration is controlled by the availability of ADP.

If we carry out work, ATP is converted to ADP to allow for more respiration which in turn will renew the storage of ATP. Under certain conditions, it will be seen that the concentration of inorganic phosphate can also affect the speed of work of the respiratory chain.

As respiration increases (as occurs during exercise), cells will approach status 3 or 5 if the respiration chain capacity becomes saturated, there is also the possibility that ADP / ATP transporters that facilitate the entry of cytosolic ADPs into and ATPs outside the mitochondria become a determinant speed of mitochondrial respiration.



The citric acid cycle or the Krebs cycle is a series of chemical reactions in the mitochondria, which take place sequentially and repeatedly, the purpose of which is to convert pyruvic acid to CO2, H2O and some energy. This process is an oxidation process using oxygen or aerobes.

Chemical reactions related to the citric acid cycle as well as reactions in the cycle itself, namely: Formation of acetyl Co-A, Formation of isocitic acid, Formation of succinyl Co-A, Formation of succinic acid, Formation of Oxaloacetic acid.

Mitochondria contain a series of catalysts known as respiration chains, which collect, transport equivalent reducing elements and direct the reaction with oxygen to form water. ATP is a free energy exchange agent, which processes exergonic processes with endergonic processes.

Enzyme complexes in the respiratory chain use the energy potential of the proton gradient to synthesize ATP from ADP and Pi. Thus it is clear that the oxidation reaction sequence is closely linked to the Krebs cycle. There are special transport proteins for the crossing of several ions and metabolites in the mitochondrial membrane.

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