Krebs Cycle | Cellular respiration

It was named after the Hens Adolf Krebs who discovered it in 1937.

It is also known by several other name citric acid cycle, Tricarboxylic acid cycle (TCA).

Citric acid cycle or TCA cycle or krebs cycle essentially involves the oxidation of acetyl CoA to Co2 & H2O.

TCA cycle is the most important central pathway connecting almost all the individual metabolic pathway.

Reactions occur in mitochondria matrix in close proximity to the E.T.C.

The process of krebs cycle are completed in following steps.

STEPS OF KREB'S CYCLE

In first step, 2 carbon molecule acetyl CoA join with 4 carbon molecule, oxaloacetate, realeasing a CoA group and farming a six carbon molecule citrate.

In second step, citrate change in its isomers isocitrate.

In third step, isocitrate is oxidized and releasing a molecule of CO2, and farming a five carbon molecule α-ketoglutarate. In this step $NAD^{+}$ reduced to form NADH. The enzyme isocitrate dehydrogenase catalysing this step.

In step four, α-ketoglutarate is oxidized and releasing a molecule of CO2, and farming a 4 carbon molecule. This 4 carbon molecule picks up a CoA and farming a unstable molecule Succinyl CoA. The enzyme α-ketoglutarate dehydrogenase catalyzing this step.

In step five, CoA of succinyl CoA is replaced by phosphate group, which is then transfered to ADP to make ATP. In some cells GDP( Guanosine diphosphate ) is used instead of ADP, forming GTP (Guanosine triphosphate) as a product. Four carbon molecule succinate is produced.

Kreb's Cycle

In six step, succinate is oxidized, forming a another four-carbon molecule called fumarate. In this reaction, two hydrogen atoms with their electrons are transferred to

\text{FAD}

, and producing

\text{FADH}_2

. The enzyme that carries out this step is embedded in inner membrane of mitochondrion, so

\text{FADH}_2

can transfer their electrons directly into the electron transport chain.

In seven step water is added to the four-carbon molecule fumarate converting it into another four-carbon molecule called malate.

In eight step, oxaloacetate the starting four carbon compound is regenerated by oxidation of malate. Another molecule of

\text{NAD}^+

reduced to form NADH.

Products of the citric acid cycle

Let’s take a step back and do some accounting tracing the fate of the carbons that enter the citric acid cycle and counting the reduced electron carriers

\text{NADH}

and

\text{FADH}_2

and

\text{ATP}

produced.

IN SINGLE TURN OF THE CYCLE

Three molecules of

\text{NADH}

NADH and one molecule of

\text{FADH}_2

are generated

One molecule of

\text{ATP}

GTP is produced.

Two carbons enter from acetyl CoA and two molecules of carbon dioxide are released.

These figures are for one turn of the cycle corresponding to one molecule of acetyl

\text{CoA}

. Each glucose produces two acetyl

\text{CoA}

molecules so we need to multiply these numbers by

2

if we want the per-glucose yield.

Two carbons from acetyl

\text{CoA}

enter the citric acid cycle in each turn and two carbon dioxide molecules are released. However the carbon dioxide molecules don’t actually contain carbon atoms from the acetyl

\text{CoA}

that just entered the cycle. Instead the carbons from acetyl

\text{CoA}

are initially incorporated into the intermediates of the cycle and are released as carbon dioxide only during later turns. After enough turns all the carbon atoms from the acetyl group of acetyl

\text{CoA}

will be released as carbon dioxide.

Where is all the A.T.P?

You may be thinking that the

\text{ATP}

out put of the citric acid cycle seems pretty unimpressive. All that work for just one

\text{ATP}

\text{GTP}

It is true that the citric acid cycle does not produce much

\text{ATP}

directly. However it can make a lot of

\text{ATP}

indirectly by way of the

\text{NADH}

and

\text{FADH}_2

it generates. All these electron carriers will connect with the last portion of cellular respiration Depositing their electrons into the electron transport chain to drive synthesis of ATP molecules through oxidative phosphorylation.