CHAPTER 4: THE CITRIC ACID AND OXIDATIVE PHOSPHORYLATION ppt

Citric Acid Cycle INTRODUCTION The citric acid cycle is a central metabolic pathway that completes the oxidative degradation of fatty acids, amino acids, and monosaccharides.. It is dur

CHAPTER 4: THE CITRIC ACID AND OXIDATIVE PHOSPHORYLATION

INTERNATIONAL UNIVERSITY SCHOOL OF BIOTECHNOLOGY

BIOCHEMISTRY

Learning objectives

1. To understand the intermediates in CAC

2. The ATPs produce in CAC

3. The CO2 is released in CAC

4. The electrons are transferred in the electron

transport chain

5. The oxidative phosphorylation

Reactants & products

Cyclical reaction pathway

Fate of acetyl CoA carbon

Introduction to the transport chain

Comlex I- comlex II- comlex III and Cytochrome C- comlex IV

Citric Acid Cycle

INTRODUCTION

The citric acid cycle is a central metabolic pathway that completes the oxidative degradation of fatty acids, amino acids, and monosaccharides During aerobic catabolism, these biomolecules are broken down to smaller molecules that ultimately contribute to a cell’s energetic or molecular needs

FIG 01: Citric acid cycle is the central metabolic pathway

Early metabolic steps, including glycolysis and the activity of the pyruvate dehydrogenase complex, yield a two-carbon fragment called an acetyl group, which is linked to a large cofactor known as coenzyme A (or CoA) It is during the citric acid cycle that acetyl-CoA is oxidized to the waste product, carbon dioxide, along with the reduction of the cofactors NAD+ and ubiquinone

FIG 02: Early catabolic pathway

FIG 03: Citric acid cycle is the central metabolic pathway

The citric acid cycle serves two main purposes:

1.To increase the cell’s ATP-producing potential by generating a reduced electron carriers such as NADH and reduced ubiquinone; and

2.To provide the cell with a variety of metabolic precursors

FIG 04: Main purposes of CAC

Understand the energetic output of the citric acid cycle;

Describe the role of the reduced electron carriers and their role in coupling the citric acid cycle to downstream reactions that produce ATP;

Describe the amphibolic character of the citric acid cycle; and Understand the reactions that replenish citric acid cycle intermediates

CELLULAR LOCATION

Both prokaryotic and eukaryotic cells use the citric acid cycle to help meet their energetic and molecular needs In respiring prokaryotes, the citric acid cycle takes place in the cytosol In eukaryotic cells, such

as the cells of the human body, the cycle takes place within the mitochondrial matrix

FIG 05: CAC can be found in both prokaryote and eukaryote

FIG 06: Matrix is where the CAC happens

The reactions of the citric acid cycle oxidize CoA’s acetyl group to two molecules of carbon dioxide During the reaction cycle, electrons are transferred from acetyl-CoA to electron carriers Once an electron carrier accepts an electron, it is referred to as “reduced.” Ultimately, reduced electron carriers participate in downstream reaction pathways that generate ATP, the energy currency of the cell

FIG 07: The products from CAC

ANABOLISM AND CATABOLISM

In addition to catabolizing molecules to meet cellular energetic needs, the citric acid cycle is key in supplying various biochemical pathways with precursors needed for synthesizing molecules Reactions that involve “building” molecules from smaller parts are referred to as anabolic Anabolic reactions use citric acid cycle intermediates as precursors for fatty acid, amino acid, and carbohydrate synthesis These anabolic processes may also require reduced cofactors

FIG 08: Intermediates in CAC

FIG 09: Intermediates in CAC are the precursors for biosynthesis

ANABOLISM AND CATABOLISM

Many citric acid cycle intermediates serve the cell as both reaction precursors and reaction products For example, a-ketoglutarate may act as a precursor for amino acids in an anabolic pathway, or it may be catabolized to carbon dioxide during the reactions of the citric acid cycle As such, the citric acid cycle is neither purely anabolic nor purely catabolic Reactions that possess this dual character of building and degrading molecules are considered amphibolic Amphi is a Greek prefix meaning both

FIG 10: The CAC is amphibolic

SOURCES OF ACETYL-CoA

The skeleton drawings of the monosaccharide glucose, the fatty acid palmitic acid, and the amino acids lysine and glutamate are depicted These molecules are degraded to a common compound called acetyl-CoA, the initiator of the citric acid cycle Select the various molecules to learn how each compound ultimately enters the citric acid cycle as acetyl-CoA Then consider how the efficiency of metabolism would change if a common product of carbohydrate, fatty acid, and amino acid catabolism did not exist

b oxidation ensue A fatty acid shortened by two carbon atoms plus a free acetyl-CoA molecule results from each round of b oxidation Acetyl-CoA initiates the citric acid cycle.

Amino Acids

Examine the structures of glutamate and lysine Recall that an amino acid consists of an amino and a carboxyl group at opposite ends, plus an attached side chain In the case of starvation, protein degradation increases and the free amino acids that result may be used as a source of metabolic fuel Alternatively, if an organism’s intake of free amino acids exceeds its protein-building needs, the free amino acids are metabolized, for there

is no storage mechanism for excess amino acids

Amino Acids

Typically, the amino group of an amino acid is removed in a deamination reaction The remaining carbon skeleton is broken down to various products depending on which of the twenty amino acids is undergoing catabolism In some cases, the remaining carbon skeleton is broken down to acetyl-CoA or to pyruvate, which is then converted to acetyl-CoA Alternatively, a citric acid cycle intermediate such as a-ketoglutarate may result In all cases, the citric acid cycle plays a large role in breaking down the amino acid skeleton to carbon dioxide

Amino Acids

For example, catabolism of lysine yeilds carbon dioxide and acetyl-CoA, while glutamate breaks down to a-ketoglutarate, carbon dioxide, and acetyl-CoA Acetyl-CoA initiates the citric acid cycle

The monosaccharide glucose is a carbon sugar In the case of higher eukaryotes, a cell most commonly acquires glucose in two ways—by breaking down complex carbohydrates into simple sugars and by mobilizing glucose from glycogen, the body’s storage system for glucose.

In the cytosol, glucose is broken down to two, carbon molecules during glycolysis The resulting three-carbon molecules are called pyruvate Pyruvate is transported across the mitochondrial membrane where it is broken down to a 2-carbon compound called acetyl-CoA plus carbon dioxide Acetyl-CoA initiates the citric acid cycle

3-FIG 11: acetyl-CoA is the initiator of CAC

REACTANTS AND PRODUCTS

Acetyl-CoA is further oxidized in the citric acid cycle As you learn each step in the reaction cycle, keep in mind that additional substrates are necessary to complete one full turn of the reaction cycle, including one GDP, one inorganic phosphate, three NAD+, and one ubiquinone, commonly referred to as, Q.

REACTANTS AND PRODUCTS

Products that emerge from one turn of the citric acid cycle are two carbon dioxide molecules, one CoA, one GTP, three NADH, and one reduced ubiquinone, referred to as QH2

FIG 12: reactants and products in CAC

CYCLICAL REACTION PATHWAY

We will now examine each of the eight reactions that make up the citric acid cycle Consider this screen your

“home base” for the reaction cycle as you study each of the eight reactions in more detail Investigate each reaction by clicking on its reaction number

FIG 13: Intermediates in CAC should be remembered

FATE OF ACETYL-COA CARBON

You have learned that acetyl-CoA and other reaction intermediates lose electrons in a series of oxidation reactions during the citric acid cycle For every acetyl-CoA molecule that enters the citric acid cycle, a total of four pairs of electrons are lost to electron carriers during the oxidation of two carbon atoms These two oxidized carbon atoms are released as two molecules

of carbon dioxide

FATE OF ACETYL-COA CARBON

Two carbon atoms are released as carbon dioxide during one round of the citric acid cycle These carbon atoms do not originate from the acetyl-CoA molecule that initiated the reaction cycle Acetyl carbons are released during subsequent rounds of the circular pathway

Trace the fate of acetyl-CoA carbon atoms through two rounds of the citric acid cycle, paying particular attention to which carbon atoms are oxidized, and when

FIG 14: Fate of acetyl-CoA carbon

FIG 15: CO 2 are released in CAC

FIG 16: CO 2 are not released from Acetyl - CoA

FIG 17: Tracking carbon

REGULATION: INHIBITION

The body functions like a finely tuned machine because its internal activities are coordinated and regulated For example, after you finishes a heavy meal, your stomach swells and stretch receptors lining the stomach send messages to your brain queueing you to, “stop eating!” On a much smaller scale, after a buildup of citric acid cycle products and intermediates accumulate, these compounds affect enzyme activity near and far, to greatly decrease cycling of the citric acid reactions

REGULATION: INHIBITION

Progression through the citric acid cycle is illustrated Notice that with each successive cycle, the levels of key regulatory compounds increase The key regulatory compounds that act to decrease the level

of citric acid cycle activity are citrate, NADH, and succinyl-CoA Collectively, these regulatory compounds function as inhibitors of the citric acid cycle

FIG 19: Regulation inhibition

FIG 20: CAC inhibition

REGULATION: ACTIVATION

In contrast to the inhibitory control previously described, positive regulators, called activators, function to up-regulate the activity of the citric acid cycle when the cell’s energetic or molecular needs are not met Key activators include Ca2+ and ADP, which signal to increase the activity of isocitrate dehydrogenase and a-ketoglutarate dehydrogenase Ca2+ and ADP generally signify the need to generate cellular free energy

FIG 21: Regulation: activation

FIG 22: Regulation: activation in CMC

During the 1860s, Louis Pasteur conducted a series

of experiments to determine if the rate of glucose metabolism is dependent on the presence or absence of oxygen His experimental set-up was similar to that shown on your screen, and his findings were coined the Pasteur effect Take a look

at the two yeast cells illustrated One is living in an aerobic environment, the other in an anaerobic environment

FIG 23: Enegetics

When a cell living under anaerobic conditions is working to meet its metabolic needs, the reactions of glycolysis are turned on For every glucose molecule consumed, the cell produces a net of two ATPs and two reduced NADH electron carriers during glycolysis In the absence of oxygen, however, oxidative phosphorylation can not take place Therefore this metabolic pathway largely responsible for transferring electrons from NADH to molecular oxygen to produce ATP does not exist Therefore, under anaerobic conditions, energy is derived solely from ATP produced during glycolysis

FIG 24: Energetics in anaerobic conditions

In a cell living in aerobic conditions, ATP is generated by three metabolic pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation Reduced electron carriers that emerge from glycolysis and the citric acid cycle are funneled to the electron transport chain, where they participate in a series

of oxidation and reduction reactions This establishes a proton gradient that spans the inner mitochondrial membrane, which ultimately drives the oxidative phosphorylation of ADP to ATP Therefore, under aerobic conditions, NADH and reduced ubiquinone, or QH2, serve the cell by increasing its ATP- producing potential

FIG 25: Energetics in aerobic conditions

ANAPLEROTIC REACTIONS

Imagine a muscle cell that is beginning to deplete its energy stores due to vigorous exercise The rate of glucose consumption increases along with a pooling

of the glycolytic product, pyruvate If molecular oxygen remains readily available, pyruvate is converted to acetyl-CoA, thereby activating the citric acid cycle The activities of citric acid enzymes are up-regulated, in large part, by an increase in the levels of the activators, calcium ions, and ADP

ANAPLEROTIC REACTIONS

Reactions exist to replenish the cell with citric acid cycle intermediates, which is especially important when metabolic activity increases, as in the case of vigorous exercise The catabolism of three types of compounds “feed” the citric acid cycle at different points Reactions that “feed” the citric acid cycle with intermediates are called anaplerotic

FIG 26: Anaplerotic reactions

FIG 27: Review of CAC

Oxidative Phosphorylation

INTRODUCTION

Oxidative phosphorylation is the process by which electrons from the reduced cofactors NADH and ubiquinol are funneled in a stepwise manner to oxygen Electrons flow much like electricity through a circuit, with free energy being conserved through the concomitant formation of a proton gradient In the end the investment of reduced cofactors results in the production of ATP

Recall that the reduced electron carriers NADH and ubiquinol are produced during glycolysis and the citric acid cycle, as well as fatty acid oxidation pathways During the cellular process of respiration, oxidative phosphorylation utilizes the chemical energy of these reduced molecules to produce ATP In nearly all eukaryotes, the ultimate electron acceptor in a series of oxidation-reduction reactions is oxygen within the mitochondrion

FIG 28: Electron transport chain

FIG 29: Electron transport chain

MITOCHONDRIAL ANATOMY

The anatomy of a mitochondrion reflects its role in the process of oxidative phosphorylation Click on the parts

of the organelle to learn about its features

The mitochondrion consists of two membranes that are separated by the intermembran space During oxidative phosphorylation, protons are pumped into this compartment