Please answer all questions fully and explain 1.How do varying levels of CO2 aff

Question

Please answer all questions fully and explain

1.How do varying levels of CO2 affect the TCA process?

2.Be familiar with anaplerotic reactions and where they occur/how they occur

3.What is oxidative phosphorylation? Where does it occur?

4.Describe the process of beta-oxidation of saturated fatty acids.

5.What is the purpose of beta-oxidation of saturated fatty acids?

6.Compare and contrast oxidation of saturated fatty acids with that of unsaturated fatty acid.

7.What are the extra steps required for monounsaturated and polyunsaturated (as compared to saturated fatty acid)?

8.How is beta oxidation linked to TCA? What are the energetics of beta-oxidation of fatty acids?

Explanation / Answer

1) The citric acid cycle (CAC) – also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle[1][2] – is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and chemical energy in the form of adenosine triphosphate (ATP).

The citric acid cycle is a key metabolic pathway that connects carbohydrate, fat, and protein metabolism. The reactions of the cycle are carried out by eight enzymes that completely oxidize acetate, in the form of acetyl-CoA, into two molecules each of carbon dioxide and water. Through catabolism of sugars, fats, and proteins, the two-carbon organic product acetyl-CoA (a form of acetate) is produced which enters the citric acid cycle. The reactions of the cycle also convert three equivalents of nicotinamide adenine dinucleotide (NAD+) into three equivalents of reduced NAD+ (NADH), one equivalent of flavin adenine dinucleotide (FAD) into one equivalent of FADH2, and one equivalent each of guanosine diphosphate (GDP) and inorganic phosphate (Pi) into one equivalent of guanosine triphosphate (GTP). The NADH and FADH2 generated by the citric acid cycle are, in turn, used by the oxidative phosphorylation pathway to generate energy-rich ATP.

One of the primary sources of acetyl-CoA is from the breakdown of sugars by glycolysis which yield pyruvate that in turn is decarboxylated by the enzyme pyruvate dehydrogenase generating acetyl-CoA according to the following reaction scheme:

CH3C(=O)C(=O)O

pyruvate

+ HSCoA + NAD+

CH3C(=O)SCoA

acetyl-CoA

+ NADH + CO2

The product of this reaction, acetyl-CoA, is the starting point for the citric acid cycle. Acetyl-CoA may also be obtained from the oxidation of fatty acids. Below is a schematic outline of the cycle:

The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate).

The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO2. The carbons lost as CO2 originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO2 requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they might not be lost, since many citric acid cycle intermediates are also used as precursors for the biosynthesis of other molecules.

Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced.

Electrons are also transferred to the electron acceptor Q, forming QH2 (Q = FAD+, QH2 = FADH2).

For every NADH and FADH2 that are produced in the citric acid cycle, 2.5 and 1.5 ATP molecules are generated in oxidative phosphorylation, respectively.

At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.

2) Anaplerotic reactions are chemical reactions that form intermediates of a metabolic pathway. Examples of such are found in the citric acid cycle (TCA cycle). In normal function of this cycle for respiration, concentrations of TCA intermediates remain constant; however, many biosynthetic reactions also use these molecules as a substrate. Anaplerosis is the act of replenishing TCA cycle intermediates that have been extracted for biosynthesis (in what are called cataplerotic reactions).

There are 4 major reactions classed as anaplerotic, and it is estimated that the production of oxaloacetate from pyruvate has the most physiologic importance.

From To Reaction Notes

Pyruvate oxaloacetate pyruvate + HCO3 + ATP {displaystyle longrightarrow } oxaloacetate + ADP + Pi + H2O This reaction is catalysed by pyruvate carboxylase, an enzyme activated by acetyl-CoA, indicating a lack of oxaloacetate. It occurs in animal mitochondria. Most important anaplerotic reaction; depending on severity, deficiency causes lactic acidosis, severe psychomotor deficiency or death in infancy

Aspartate oxaloacetate - This is a reversible reaction forming oxaloacetate from aspartate in a transamination reaction, via aspartate transaminase.

Glutamate -ketoglutarate glutamate + NAD+ + H2O {displaystyle longrightarrow } NH4+ + -ketoglutarate + NADH + H+. This reaction is catalysed by glutamate-dehydrogenase.

-Oxidation of fatty acids succinyl-CoA - When odd-chain fatty acids are oxidized, one molecule of succinyl-CoA is formed per fatty acid. The final enzyme is methylmalonyl-CoA mutase. Triheptanoin (fat with three heptanoic (C7:0) fatty acids) may be used to treat pyruvate carboxylase deficiency

adenylosuccinate fumarate adenylosuccinate {displaystyle longrightarrow } AMP + fumarate This reaction is catalysed by adenylosuccinate lyase and occurs in purine synthesis and purine nucleotide cycle. Defect of this enzyme causes psychomotor retardation.

The malate is created by PEP carboxylase and malate dehydrogenase in the cytosol. Malate, in the mitochondrial matrix, can be used to make pyruvate (catalyzed by malic enzyme) or oxaloacetic acid, both of which can enter the citric acid cycle.

Glutamine can also be used to produce oxaloacetate during anaplerotic reactions in various cell types through "glutaminolysis", which is also seen in many c-Myc transformed cells.

4) n biochemistry and metabolism, beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cycle, and NADH and FADH2, which are co-enzymes used in the electron transport chain. It is named as such because the beta carbon of the fatty acid undergoes oxidation to a carbonyl group. Beta-oxidation is primarily facilitated by the mitochondrial trifunctional protein, an enzyme complex associated with the inner mitochondrial membrane, although some fatty acids are oxidized in peroxisomes.

The overall reaction for one cycle of beta oxidation is:

Cn-acyl CoA + FAD + NAD+

+ H

2O + CoA

Cn-2-acyl CoA + FADH

2 + NADH + H+

+ acetyl CoA

Fatty acid catabolism consists of:

Activation and membrane transport of free fatty acids by binding to coenzyme A.

Oxidation of the beta carbon to a carbonyl group.

Cleavage of two-carbon segments resulting in acetyl-CoA.

Oxidation of acetyl-CoA to carbon dioxide in the citric acid cycle.

Electron transfer from electron carriers to the electron transport chain in oxidative phosphorylation.

Once inside the mitochondria, each cycle of -oxidation, liberating a two carbon unit (acetyl-CoA), occurs in a sequence of four reactions:

Description Diagram Enzyme End product

Dehydrogenation by FAD: The first step is the oxidation of the fatty acid by Acyl-CoA-Dehydrogenase. The enzyme catalyzes the formation of a double bond between the C-2 and C-3.

acyl CoA dehydrogenase trans-2-enoyl-CoA

Hydration: The next step is the hydration of the bond between C-2 and C-3. The reaction is stereospecific, forming only the L isomer.

enoyl CoA hydratase L--hydroxyacyl CoA

Oxidation by NAD+: The third step is the oxidation of L--hydroxyacyl CoA by NAD+. This converts the hydroxyl group into a keto group.

3-hydroxyacyl-CoA dehydrogenase -ketoacyl CoA

Thiolysis: The final step is the cleavage of -ketoacyl CoA by the thiol group of another molecule of Coenzyme A. The thiol is inserted between C-2 and C-3.

-ketothiolase An acetyl-CoA molecule, and an acyl-CoA molecule that is two carbons shorter

This process continues until the entire chain is cleaved into acetyl CoA units. The final cycle produces two separate acetyl CoAs, instead of one acyl CoA and one acetyl CoA. For every cycle, the Acyl CoA unit is shortened by two carbon atoms. Concomitantly, one molecule of FADH2, NADH and acetyl CoA are formed.

3)Oxidative phosphorylations the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing energy which is used to produce adenosine triphosphate (ATP). In most eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is probably so pervasive because it is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.

During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors such as oxygen, in redox reactions. These redox reactions release energy, which is used to form ATP. In eukaryotes, these redox reactions are carried out by a series of protein complexes within the inner membrane of the cell's mitochondria, whereas, in prokaryotes, these proteins are located in the cells' intermembrane space. These linked sets of proteins are called electron transport chains. In eukaryotes, five main protein complexes are involved, whereas in prokaryotes many different enzymes are present, using a variety of electron donors and acceptors.

The energy released by electrons flowing through this electron transport chain is used to transport protons across the inner mitochondrial membrane, in a process called electron transport. This generates potential energy in the form of a pH gradient and an electrical potential across this membrane. This store of energy is tapped when protons flow back across the membrane and down the potential energy gradient, through a large enzyme called ATP synthase; this process is known as chemiosmosis. The ATP synthase uses the energy to transform adenosine diphosphate (ADP) into adenosine triphosphate, in a phosphorylation reaction. The reaction is driven by the proton flow, which forces the rotation of a part of the enzyme; the ATP synthase is a rotary mechanical motor.

Although oxidative phosphorylation is a vital part of metabolism, it produces reactive oxygen species such as superoxide and hydrogen peroxide, which lead to propagation of free radicals, damaging cells and contributing to disease and, possibly, aging (senescence). The enzymes carrying out this metabolic pathway are also the target of many drugs and poisons that inhibit their activities.

It is the terminal process of cellular respiration in eukaryotes and accounts for high ATP yield.

5)

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