Biochem Lecture 26- Citric Acid Cycle Part 3

How is TCA cycle completed?

By the regeneration of oxaloacetate from succinate in several steps

What is the full equation for TCA cycle?

Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O → 2CO2 + CoA-SH + 3NADH + 3H+ + FADH2 + GTP

What is the overall reaction for the oxidation of pyruvate (forms CO2 and ATP)

Pyruvate + 4NAD+ + FAD + GDP + Pi + 2H2O → 3CO2 + 4NADH + FADH2 + GTP + 4H+

What three reaction in TCA cycle are highly exergonic?

-Pyruvate dehydrogenase (before step 1)

-citrate synthase (step 1)

-a-ketoglutarate dehydrogenase (step 4)

What reaction in TCA cycle is highly endergonic?

Malatate dehydrogenase (step 8)

What steps within the cell of TCA cycle are regulatory enzymes?

-citrate synthase (step 1)

-isocitrate dehydrogenase (step 3)

-a-ketoglutarate dehydrogenase complex (step 4)

What steps of TCA cycle outside the cell are regulatory enzymes?

Pyruvate dehydrogenase (before step 1)

What does a highly active metabolic state equal?

A low energy charge because energy demand depletes ATP and NADH with both ATP/ADP and NADH/NAD+ ratios low (increases oxidative reactions)

What does a resting metabolic state equal?

A high energy charge because there are abundant levels of ATP and NADH with both ATP/ADP and NADH/NAD+ ratios high (slows down oxidative reactions)

Cells in a resting metabolic state:

-Need/use very little energy

-have high ATP and low ADP levels= high ATP/ADP

-high NADH and low NAD+ levels= high NADH/NAD+

Cells in a highly active metabolic state

-need/use more energy than resting cells

-have low ATP and high ADP= low ATP/ADP levels

-have low NADH and high NAD+= low NADH/NAD+ levels

Where does the conversion of pyruvate to acetyl-CoA take place?

in BOTH plants and animals

Why can pyruvate not be produced from acteyl-CoA in mammals?

Because acetyl-CoA is the end product of catabolism of fatty acids

What carries out the conversion of acetyl-CoA to pyruvate and oxaloacetate?

Plants; they can exist without carbohydrates as a carbon source

Glyoxylate cycle

Pathway in plants and some bacteria that is an alternative to TCA cycle. Permits acetyl-CoA to enter Gluconeogenesis and bypasses the to oxidative decarboxylation steps of TCA cycle

What is the purpose of the glyoxylate pathway?

It routes isocitrate via glyoxylate to malate

Isocitrate lyase

Catalyzes the cleave of isocitrate to glyoxylate and succinate

Malate synthase

Catalyzes the reaction of glyoxylate (CHO-COOH) with acetyl-CoA to produce malate

Where does the glyoxylate cycle take place?

In glyoxysomes and the cytoplasm of yeast and algae

Glyoxysomes

Specialized organelles in plants

What does the glyoxylate cycle help with?

It helps with the growth of plants in the dark. During germination plants use the acetyl-CoA produced in fatty acid oxidation to produce oxaloacetate and other intermediates for carbohydrate synthesis.

What happens to glyoxysomes when plants can begin to photosynthesis again?

They disappear

In catabolism how do nutrients (Macromolecules) enter TCA cycle?

They are broken down and are converted from larger (polysaccharides, lipids) to smaller molecules (sugars, amino acids)

Where do catabolic reactions occur?

In the cytosol and the end products of catabolism cross the mitochondrial membrane

Where does TCA cycle primarily occurs?

The mitochondrial matrix