Citric Acid Cycle-with Quiz Biochemistry

7/28/2019 Citric Acid Cycle-with Quiz Biochemistry

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Also known as the TCA cycle

or Krebs cycle

CITRIC ACID CYCLE

Group 4

:: Dote, Nikko :: Villaver, Stephanie :: Quitorio, Freah

:: Abella, Tiffany :: Enriquez, Paola Jane :: Dipaling, Nio Albert:: Habana, Carolyn


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is a series of chemical reactions used

by all aerobic organisms to generateenergy through the oxidization of

acetate derived from carbohydrates,

fats and proteins into carbon dioxide.a series of enzyme-catalyzed

chemical reactions that form a key

part of aerobic respiration in cells


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In addition,

the cycle provides precursors includingcertain amino acids as well as the reducing

agent NADH that is used in numerous

biochemical reactions Its central importance to many biochemical

pathways suggests that it was one of the

earliest established components of cellularmetabolism and may have originated

abiogenically.


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Origin of the cycles name

The name of this metabolic pathway is

derived from citric acid (a type of

tricarboxylic acid) that is first

consumed and then regenerated by

this sequence of reactions to complete

the cycle.


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Evolution

Components of the TCA cycle were derivedfrom anaerobic bacteria, and the TCA cycle

itself may have evolved more than once.

Theoretically there are several alternativesto the TCA cycle, however the TCA cycle

appears to be the most efficient. If several

TCA alternatives had independentlyevolved, they all appear to have converged

onto the canonical TCA cycle.


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Definition of terms

Pyruvate-

Pyruvate is the end product of glycolysis,

which is converted into acetyl coA that entersthe Krebs cycle when there

is sufficient oxygen available. When

the oxygen is insufficient, pyruvate is brokendown anaerobically,

creating lactate in animals (including humans)

and ethanol in plants.


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Acetyl-CoA

Acetyl coenzyme A oracetyl-CoA is animportant molecule in metabolism, used in

many biochemical reactions. Its main function

is to convey the carbon atoms within

the acetyl group to the citric acid cycle (Krebs

cycle) to be oxidized for energy production.

Definition of terms


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FAD - flavin adenine dinucleotide (FAD)

In biochemistry, is a redox cofactor involved in

several important reactions in metabolism. FAD

can exist in two different redox states, which itconverts between by accepting or donating

electrons. The molecule consists of a

riboflavin moiety (vitamin B2) bound to the

phosphate group of an ADP moleculeFAD can be reduced to FADH2, whereby it accepts

two hydrogen atoms (a net gain of two electrons):

Definition of terms


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NADH- Nicotinamide adenine dinucleotide, - abbreviated NAD+, is a coenzyme found in all

living cells.

- In metabolism, NAD+ is involved in redox

reactions, carrying electrons from one reaction toanother.

- The coenzyme is, therefore, found in two forms in

cells: NAD+ is an oxidizing agent it accepts

electrons from other molecules and becomesreduced. This reaction forms NADH, which can then

be used as a reducing agent to donate electrons.

These electron transfer reactions are the main

function of NAD+.

Definition of terms


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The cycleoverview


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1. a two carbon organic product acetate in the form of acetyl-

CoA is produced through catabolism of sugars, fats, and

proteins.

2. Acetyl-CoA along with two equivalents of water (H2O) isconsumed by the citric acid cycle producing two

equivalents of carbon dioxide (CO2) and one equivalent of

HS-CoA.

3. one complete turn of the cycle converts three equivalentsof nicotinamide adenine dinucleotide (NAD+) into three

equivalents of reduced NAD+ (NADH), one equivalent of

ubiquinone (Q) into one equivalent of reduced ubiquinone

(QH2), and one equivalent each of guanosine diphosphate

(GDP) and inorganic phosphate (Pi) into one equivalent ofguanosine triphosphate (GTP).

4. The NADH and QH2 generated by the citric acid cycle are

in turn used by the oxidative phosphorylation pathway to

generate energy-rich adenosine triphosphate (ATP).


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One of the primary sources of acetyl-CoA is sugars

that are broken down by glycolysis to produce 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


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The product of this reaction,

acetyl-CoA, is the starting point for

the citric acid cycle.


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During glycolysis, glucose isbroken down to pyruvate.

A two-carbon fragment ofpyruvate is used to form acetyl-CoA. The acetyl CoAenters the Krebs Cycle, which occurs in the mitochondrion.

During the conversion ofpyruvate to acetyl-CoA, carbon dioxide (CO2) is produced and a

molecule of NADH is formed

The two carbon acetyl portion of the acetyl-CoA is transferred to a

4-carbon molecule, producing a six-carbon compound. The CoA

carrier molecule is released.

Carbon dioxide is then released from the 6-carbon molecule, forming a 5-

carbon compound. In this step, hydrogen is removed and transferred to

NAD+ to form NADH


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Finally the 4-carbon molecule is further oxidized and the hydrogens that

are removed are used to form NADH and FADH2. These reactionsregenerate the 4-carbon molecule that initially reacts with acetyl-CoA.

Each glucose molecule is broken down into two pyruvate molecules

during glycolysis. Then each pyruvate is converted to acetyl-CoA, whichenters the Krebs Cycle.

Thus, for each glucose molecule, the Krebs cycle must complete two

circuits to completely break dwon the two pyruvate molecules.

Next, a second oxidation and decarboxylation occurs. Again, NADH and carbon

diocide are produced. In addtition, a molecule of ATP is produced. As a result ofthese reactions, a 4-carbon molecule is formed in the Krebs Cycle.


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Simplified


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2 only per cylce


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The greatly simplified cycle below starts withpyruvate, which is the end product of gylcolysis, thefirst step of all types of cell respiration.

Since it is well known that ATP is the energy currencythat is used for nearly every task in the human body

and other organisms, it may seem surprising that solittle ATP is produced directly by the TCA cycle. Butthe TCA cycle as shown here represents the processtaking place in the matrix of the mitochondria usingthe reactions with NAD+ and FAD which carry highenergy electrons to the electron transport systemsinthe inner mitochondrial membrane. The reactionsthere are responsible for the production of most of the

ATP yield by chemiosmosis.


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The Krebs cycle, also called the citric acid

cycle, is a fundamental metabolic pathwayinvolving eight enzymes essential for energy

production through aerobic respiration, and,

like glycolysis, arose early in evolution

The Krebs cycle takes place in mitochondriawhere it oxidizes acetyl-CoA, releasing carbon

dioxide and extracting energy primarily as the

reduced high-energy electron carriers NADH

and FADH2. NADH and FADH2 transferchemical energy from metabolic intermediates

to the electron transport chain to create a

different form of energy, a gradient of protons

across the inner mitochondrial membrane.


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The Krebs cycle, like other metabolic pathways,

is tightly regulated to efficiently meet the needsof the cell and the organis

Some Krebs cycle enzymes require non-protein

cofactors for activity, such as thiamine, vitamin

B1. Insufficient quantities of this vitamin in thediet leads to decreased activity of pyruvate

dehydrogenase and alpha-ketoglutarate

dehydrogenase, and a decrease in the ability of

the Krebs cycle to meet metabolic demands,causing the disease beriberi.


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Kreb's cycle is important in the processes

of cellular respiration as well as theprocessing of fats and other molecules for

the harvesting of chemical energy

Fats are broken down in a process calledbeta oxidation and enter the Kreb's cycle

as acetyl groups.

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Citric Acid Cycle-with Quiz Biochemistry