During fasting or carbohydrate starvation, oxaloacetate is depleted in liver because it is used for gluconeogenesis. This impedes entry of acetyl-CoA into Krebs cycle. Acetyl-CoA then is converted in liver mitochondria to ketone bodies, acetoacetate and b-hydroxybutyrate.

 Three enzymes are involved in synthesis of ketone bodies:

b-Ketothiolase. The final step of the b-oxidation pathway runs backwards, condensing 2 acetyl-CoA to produce acetoacetyl-CoA, with release of one CoA.

HMG-CoA Synthase catalyzes condensation of a third acetate moiety (from acetyl-CoA) with acetoacetyl-CoA to form hydroxymethylglutaryl-CoA (HMG-CoA).

HMG-CoA Lyase cleaves HMG-CoA to yield acetoacetate plus acetyl-CoA.

 b-Hydroxybutyrate Dehydrogenase catalyzes inter-conversion of the ketone bodies acetoacetate and b-hydroxybutyrate.

Ketone bodies are transported in the blood to other tissue cells, where they are converted back to acetyl-CoA for catabolism in Krebs cycle

Essential vs. Nonessential Amino Acids

*The amino acids arginine, methionine and phenylalanine are considered essential for reasons not directly related to lack of synthesis. Arginine is synthesized by mammalian cells but at a rate that is insufficient to meet the growth needs of the body and the majority that is synthesized is cleaved to form urea. Methionine is required in large amounts to produce cysteine if the latter amino acid is not adequately supplied in the diet. Similarly, phenyalanine is needed in large amounts to form tyrosine if the latter is not adequately supplied in the diet.

Functions of  lipids

1. They are the concentrated fuel reserve of the body  (triacylglycerols).

2. Lipids are the constituents of membrane structure and regulate the membrane permeability (phospholipids  and cholesterol).

3. They serve as a source of fat soluble vitamins (A, D, E and K).

4. Lipids are important as cellular metabolic regulators (steroid  hormones and prostaglandins).

5. Lipids protect the internal organs, serve as insulating materials and give shape and smooth appearance to the body.

Anaerobic organisms lack a respiratory chain. They must reoxidize NADH produced in Glycolysis through some other reaction, because NAD⁺ is needed for the Glyceraldehyde-3-phosphate Dehydrogenase reaction (see above). Usually NADH is reoxidized as pyruvate is converted to a more reduced compound, that may be excreted.

The complete pathway, including Glycolysis and the re-oxidation of NADH, is called fermentation.

For example, Lactate Dehydrogenase catalyzes reduction of the keto group in pyruvate to a hydroxyl, yielding lactate, as NADH is oxidized to NAD⁺.

Skeletal muscles ferment glucose to lactate during exercise, when aerobic metabolism cannot keep up with energy needs. Lactate released to the blood may be taken up by other tissues, or by muscle after exercise, and converted via the reversible Lactate Dehydrogenase back to pyruvate

Fermentation Pathway, from glucose to lactate (omitting H⁺):

   glucose + 2 ADP + 2 P_i → 2 lactate + 2 ATP

Anaerobic catabolism of glucose yields only 2 “high energy” bonds of ATP.

- There are two important phospholipids, Phosphatidylcholine and Phosphatidylserine found the cell membrane without which cell cannot function normally.

- Phospholipids are also important for optimal brain health as they found the cell membrane of brain cells also which help them to communicate and influence the receptors function. That is the reason food stuff which is rich in phospholipids like soy, eggs and the brain tissue of animals are good for healthy and smart brain.

- Phospholipids are the main component of cell membrane or plasma membrane. The bilayer of phospholipid molecules determine the transition of minerals, nutrients, and drugs in and out of the cell and affect various functions of them.

- As phospholipids are main component of all cell membrane, they influence a number of organs and tissues, such as the heart, blood cells and the immune system. As we grown up the amount of phospholipids decreases and reaches to decline.

- Phospholipids present in cell membrane provide cell permeability and flexibility with various substances as well its ability to move fluently. The arrangement of phospholipid molecules in lipid bilayer prevent amino acids, carbohydrates, nucleic acids, and proteins from moving across the membrane by diffusion. The lipid bi-layer is usually help to prevent adjacent molecules from sticking to each other.

- The selectivity of cell membrane form certain substances are due to the presence of hydrophobic and hydrophilic part molecules and their arrangement in bilayer. This bilayer is also maintained the normal pH of cell to keeps it functioning properly.

- Phospholipids are also useful in the treatment of memory problem associated with chronic substances as they improve the ability of organism to adapt the chronic stress.

Cholesterol synthesis:

Hydroxymethylglutaryl-coenzyme A (HMG-CoA) is the precursor for cholesterol synthesis. 

HMG-CoA is also an intermediate on the pathway for synthesis of ketone bodies from acetyl-CoA. The enzymes for ketone body production are located in the mitochondrial matrix. HMG-CoA destined for cholesterol synthesis is made by equivalent, but different, enzymes in the cytosol.

HMG-CoA is formed by condensation of acetyl-CoA and acetoacetyl-CoA, catalyzed by HMG-CoA Synthase.

HMG-CoA Reductase, the rate-determining step on the pathway for synthesis of cholesterol.