Mitochondria: Comprehensive Discussion

The Mito Comprehensive Discussion page is dedicated to helping those of you truly intrigued individuals have a broader, biological understanding of Mitochondria and the other related pre- and post- energy activities associated with metabolism and ATP-energy production.  The following concepts are complex and it is assumed that you have already familiarized yourself with the introductory information on the page “What Are Mitochondria.”

EUKARYOTIC CELL PHYSIOLOGY:

Eukaryotic cells contain a true membrane-bound nucleus that surrounds chromatin (nuclear DNA)/chromosomes and the nucleolus.  The cell has an outer plasma membrane that provides a barrier for the cell and contains transport and signaling systems (lipid, protein, carbohydrate complex).  Cytoplasm is the fluid or gel filled portion of the cell. All Eukaryotic cells contain double membrane, bacteria-like organelles called Mitochondria (plural form of Mitochondrion and Mito for short).  Mito membranes contain folds called Cristae.  Mito contain their own DNA, often referred to as MtDNA, that is separate from nuclear DNA.  Endoplasmic Reticulum provide two networks of interconnected membranes; one covered in Ribosomes which contain protein and RNA complexes for protein synthesis, the other for synthesis and metabolism of lipids and also utilizes enzymes to perform detoxifying functions.  Lysosomes are also membrane-bound organelles that act to degrade proteins, membranes and ingested materials within the cell.

HOW THE NUTRIENTS WE CONSUME ULTIMATELY BECOME ATP:

DIET:  Dietary needs continue to be a fiercely debated issue.  What is universally true is the need for a variety of high nutrient foods that can be converted into chemical compounds that cells can recognize and use.  If one or more necessary components are missing, the body will struggle to provide the energy necessary to survive.  Some components can be derived from others as the body shifts to “work-arounds.”  Other nutrients are considered “essential” because the body cannot synthesize the compounds on its own, or not in adequate amounts, and therefore must be provided as a part of a “balanced diet.”

POTENTIAL FAILURES:  Obviously, providing a lack of adequate nutritional requirements is the first  weak link that can lead to impaired Mito / metabolic function.

DIGESTION:  The nutrition provided (through both food and quality supplements) must be converted to a form that can be recognized and utilized by cells.  The process of digestion alters consumed nutrients by breaking them down into their chemical components.

POTENTIAL FAILURES:  Despite providing adequate nutritional requirements, issues with digestion can impair the absorption and utilization of nutrients being provided.

DELIVERY OF NUTRIENT CHEMICAL COMPOUNDS:  In order for the extracted carbohydrates, fats, proteins and other molecules to be utilized, they must be transported by the bloodstream to the cells.  These nutrients, now in the form of chemical compounds, leave the bloodstream through capillary walls and enter cells.  Once inside the cell, the process of utilizing these nutrients begins.

POTENTIAL FAILURES:  Delivery via bloodstream, transfer through capillary walls and cellular uptake can all play a role in whether essential compounds reach the cell to take part in the process of Cellular Respiration.

CELLULAR RESPIRATION:  How Nutrient Compounds are Utilized to Create ATP-energy

Step 1:  Glycolysis:

Glycolysis literally means “splitting sugars.”  This process is anaerobic (occurring without the presence of oxygen) and takes place in the cytoplasm of the cell.  It consists of two distinct phases; an energy investment phase and an energy harvesting phase.

Phase 1 – Energy-Investment Phase of Glycolysis:

A 6-carbon glucose molecule binds with 2 ATP molecules.  During the binding process, a Phosphate is lost from each ATP, resulting in conversion to 2 ADP.  The combination of the glucose and ADP form a 6-carbon sugar diphosphate molecule.  This molecule splits into two 3-carbon sugar diphosphate molecules known as 2-Phosphoglyceraldehyde (2PGAL), beginning the energy-harvesting phase.

Phase 2 – Energy-Harvesting Phase of Glycolysis:

2PGAL are converted.  This process takes 4 ADP, adds an additional phosphate to each, creating 4 ATP.  Also, 2 NAD+ molecules each pick up an extra electron and become 2 NADH molecules.  And the end product also includes the formation of 2 molecules of Pyruvic Acid.

Glycolysis is a ten-step reaction that involves the activity of multiple enzymes and enzyme assistants.  In the process, for each 6-carbon glucose molecule, the following are produced:

2 net ATP (a total of 4 ATP are produced, but it requires 2 ATP in the process)

2 NADH (high-energy electron carrying molecules produced through H electron donation to NAD+)

2 H+ (released as protons)

2 Pyruvate (Pyruvic Acid)

Step 2:  Intermediate / Preparatory or Linked Reaction Stage (Pyruvate Oxidation):

Following Glycolysis, when sufficient oxygen is present in the cell, Aerobic Cellular Respiration continues and the Pyruvic Acid and NADH move into the Mitochondria where the Pyruvic Acid undergoes oxidation.  During the series of chemical reactions, each Pyruvic Acid molecule loses one CO2 molecule.  It then combines with CoEnzyme A to produce a compound called Acetyl CoEnzyme A (Acetyl-CoA).

For each Pyruvate (2 Pyruvate molecules for each glucose molecule broken down through Glycolysis), the following is produced:

1 Acetyl-CoA (1 carbon is stripped from each Pyruvate, leaving the 2-carbon compound that is joined by CoEnzyme A, creating Acetyl-CoA) (2 total per glucose molecule)

1 NADH (1 H is donated to NAD+ to create NADH) (2 total per glucose molecule)

1 carbon (2 oxygen join the carbon to form 1 molecule of CO2, which is expelled) (2 total CO2 per glucose)

POTENTIAL FAILURES:  The catabolism of glucose in the cytosol produces 2 molecules of pyruvate.  These 2 pyruvate molecules are enzymatically converted into 2 molecules of acetyl-coenzyme A (acetyl CoA).  This conversion requires:

1. Coenzyme A (CoA), which is derived from pyruvic acid, pantothenic acid (vitamin B5) and cysteine

2. NAD+, which contains niacin (vitamin B3)

3. FAD+, which contains riboflavin (vitamin B2)

4. Lipoic acid

5. Thiamine pyrophosphate (TPP), which contains thiamine (vitamin B1)

Step 3:  Citric Acid Cycle (Krebs Cycle):  The next stage is the citric acid cycle, also called the Krebs cycle.  ALL PROCESSES IN THE KREBS CYCLE ARE ENZYME CATALYZED REACTIONS.

During the Krebs Cycle, the Acetyl-CoA is broken down to ultimately form high energy electron carriers, NADH and FADH2, that will now enter the Electron Transport Chain where the majority of energy is produced.  1 ATP molecule is produced for each completed Krebs Cycle, again, minimal ATP production.

For each turn of the Krebs cycle (1 turn for each Pyruvate, which is 2 Pyruvate per glucose molecule):

1 ATP (2 total, 1 for each of the 2 pyruvate molecules produced from each glucose molecule)

3 NADH (6 total, 3 for each of the 2 pyruvate molecules produced from each glucose molecule)

1 FADH2 (2 total, 1 for each of the 2 pyruvate molecules produced from each glucose molecule)

CO2 is a by-product of the Krebs Cycle

(This process is similar to the Calvin cycle phase in plants)

Acetyle-CoA will bind with a starting compound called Oxaloacatate, and through a series of enzymatic redox reactions, all carbons, hydrogens, and oxygens in Pyruvate ultimately end up as carbon dioxide and water.  The pathway is called a cycle because Oxaloacetate is the starting and ending compound of the pathway.  For every glucose that enters Glycolysis, the cycle completes twice, once for each molecule of Pyruvate that entered the Mitochondria.

In order to understand how the majority of the energy is produced by aerobic respiration, we need to follow the NADH and FADH2 molecules to the next stage, the Electron Transport Chain.  Each NADH will produce 3 ATP in the ETC, for a total of 30 ATP.  Each FADH2 will produce 2 ATP in the ETC, for a total of 4 ATP.  (This is assuming the most efficient function of cellular respiration.  Average ATP production is 32 to 36 ATP.)

In-depth breakdown of the 8-steps in the Krebs Cycle:

Step 1:  2-carbon Acetyl-CoA binds with the 4-carbon Oxaloacetate to form Citrate, and CO2 is released.

Step 2 and 3:  Citrate rearranges to form Isocitrate (the hydroxal group is moved to a new position from Citrate to Isocitrate.

Step 4:  In this reaction, Isocitrate is converted to alpha-Ketoglutarate.  NAD+ picks up one Hydrogen (H), effectively capturing energy (by receiving an electron) and forms NADH.  Another H+ is released as a proton.  The carbon and 2 oxygen molecules are released as CO2. The remaining molecular structure is alpha-Ketoglutarate.

Step 5:  alpha-Ketoglutarate is converted into succinyl CoA by the addition of CoEnzyme A.  The enzyme from this reaction adds a high energy thyoestrabond to CoA, releasing the carbon and 2 oxygen atoms and converting NAD+ to NADH and releases another CO2 molecule.

Step 6:  Succinyl CoA is converted to Succinate (a symmetrical molecule).  During this reaction, the CoA group is released and generates enough energy to convert GDP, an inorganic phosphate, to GTP, an energy carrying molecule related to ATP.

Step 7:  Succinate is converted to Fumarate.  H2 are stripped off and donated to FAD to produce a molecule of FADH2.  FADH2, like NADH, is a high-energy carrier that feeds high-energy electrons into the ETC.

Step 8:  Fumarate combines with H2O to produce Malate.

Step 9:  Malate is converted back to Oxaloacetate.  Two H molecules are released, one combining to NAD+ to create the final NADH molecule produced in the cycle.  The other, H+, is released as a proton.  The remaining molecular structure is Oxaloacetate.  The replenished Oxaloacetate can now take part in another cycle, returning to step 1 in the cycle.

POTENTIAL FAILURES:  Acetyl CoA then passes through the Mitochondria’s double membrane to enter the Kreb’s cycle.  The Kreb’s cycle has 9 steps, and its completion requires the following cofactors:

è cysteine, iron, niacin, magnesium, manganese, thiamine, riboflavin, pantothenic acid, and lipoic acid.

In the Kreb’s cycle, each acetyl-CoA produces 3 molecules of NADH and 2 molecules of FADH, for a total of 6 NADH and 4 FADH per one molecule of pyruvate.  Acetyl-CoA can be produced by oxidation of fatty acids, which then requires the nutrient l-carnitine to shuttle the acetyl-CoA into the Mitochondria to enter the Kreb’s cycle.

Step 4:  Electron Transport Chain (ETC):

All the high energy electron carriers from the previous stages of cellular respiration bring their electrons into the chain.  (NADH and FADH2 are both created by the acceptance of an electron).  As NADH and FADH2 enter into the ETC, this is how electrons are “carried” into the ETC.  As the NADH and FADH2 molecules pass through a series of proteins embedded in the Mitochondrial membrane (membrane-bound carriers in the Mitochondria), electrons are transferred between the membrane proteins and the cell is able to capture energy and use it to produce ATP molecules.  Oxygen (O2) acts as the terminal or final electron acceptor.  By accepting these electrons, oxygen binds to Hydrogen (H) and forms H2O/water, a byproduct of the ETC.  Out of the entire process of cellular respiration, the ETC produces the bulk of ATP; on average, a net of 32 to 36 ATP.

STEPS IN THE ETC:

Oxidation of NADH, breaking it down:

NAD+ (which is recycled back in future glycolysis and Krebs Cycle functions)

H+ (a proton)

2 electrons (oxidation is loosing electrons)

Electrons that are released from oxidation are in a high energy state.  They are pumped across transition molecules:

CoQ

Cytochrome C

Every time an electron goes from a high energy state, crossing the transition molecules.  As the electrons bind with each transition molecule, it releases energy.  The energy being released is used to pump protons across the Crista or inter membrane of the Mitochondria into the outer membrane where the outer membrane becomes more acidic (and positive) than the inter membrane or Matrix of the cell (and more negative).  The cellular Crista is impermeable to H+.  Past the Crista, inside the Matrix, there is ATP Synthase.  H+ can cross the ATP Synthase to drive the membrane axle inside the Matrix.  An ADP molecule will attach to the Matrix along with a free phosphate.  As the ATP Synthase axle rotates and spins, the ADP and phosphate are pushed together to form ATP.  (This form of ATP production is called Oxidative Phosphorylation.  The transfer of H+ through the ATP Synthase to cause the axle to spin and combine ADP with the phosphate to create the ATP is called Chemiosmosis.)  (When ATP is produced without Chemiosmosis, via alternative enzyme driven processes, it is called Substrate Phosphorylation.)

Last step of the ETC, reduction of oxygen to water (oxygen is the final electron acceptor):

2 electrons

2 H+

1 O

Otherwise known as H2O

POTENTIAL FAILURES:  The electron transport chain (ETC) is embedded in the inner Mitochondrial membrane and consists of a series of five enzyme complexes, designated I–V.

– NADH and FADH carry electrons to the ETC.

– Electrons donated from NADH and FADH flow through the ETC complexes, passing down an electrochemical gradient to be delivered to oxygen (O2).

– Electron transport complexes I–IV require ubiquinone (Coenzyme Q10, or CoQ10).

– Electron transport complex IV is a cytochrome (cytochrome c) enzyme.

– Electron transport complexes I–IV contain flavins, which contain riboflavin, iron, sulfur, and copper.

– CoQ10 shuttles electrons from complexes I and II to complex III.

– Cytochrome c, an iron-containing heme protein that transfers electrons from electron transport complex III to IV.

– “During this process, protons are pumped through the inner Mitochondrial membrane to the intermembrane space to establish a proton motive force, which is used by complex V to Phosphorylate Adenosine Diphosphate (ADP) by ATP synthase, thereby creating ATP.”

ANAEROBIC RESPIRATION – Alternate ATP Production in the Absence of Oxygen:  Most people are familiar with the concept of Lactic Acid, the source of muscle soreness following an intense workout.  The generation of Lactic Acid is through the process of Fermentation.

Following Step 1 of Glycolysis, if oxygen in the cell is scarce, the preferred and most efficient cellular respiration process capable of producing an average of 36 ATP cannot take place.  However, the energy demands of the body persist.  If ATP fails to be provided at a level greater than the demand, cellular death and eventual organism death will occur.  In fact, the very process of creating ATP is essential to survival.  Imagine the heart simply not having sufficient ATP-energy to sustain an adequate heart rate.  Therefore, in an effort to preserve homeostasis (survival of the cell and/or organism) the body will bypass the second, third and fourth steps of Cellular Respiration and shift to the process of Lactic Acid Fermentation.

Following Glycolysis, each molecule of NADH donates its high energy Hydrogen electron which bonds to a molecule of Pyruvic Acid forming a Lactic Acid molecule.  No ATP is generated during the process of Fermentation.  It merely replenishes the available NAD+ “Hydrogen carriers” to ensure future Glycolysis can occur (which produces a net gain of 2 ATP verses 36 ATP through Aerobic Cellular Respiration).  However, Fermentation cannot occur indefinitely.  As Lactic Acid builds up in the cytoplasm of the cell, a point of saturation is reached and the process of Fermentation will cease.  Therefore, the NADH Hydrogen carriers are not converted back to NAD+, and eventually, the Glycolysis reaction, which produces the ATP, will stop.

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