Chapter 26, Problem 13P

Interpretation Introduction

(a)

Interpretation:

The activated reactant in the biosynthesis of Phosphatidylinositol from inositol should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

CDP-diacylglycerol is the reactant during the biosynthesis of Phosphatidylinositol from inositol.

Explanation of Solution

During the phospholipid production, phosphatidate usually reacts with Cytidine triphosphate or CTP. This reaction produces CDP-diacylglycerol. Then CDP-diacylglycerol further reacts with alcohol and produce a phospholipid. Instead, diacylglycerol can also react with a Cytidine Diphosphate or CDP alcohol to synthesize a phospholipid.

So, CDP-diacylglycerol is the reactant during the biosynthesis of Phosphatidylinositol from inositol.

Interpretation Introduction

(b)

Interpretation:

The activated reactant in the biosynthesis of Phosphatidylethanolamine from ethanolamine should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

CDP-ethanolamine is the activated reactant in the biosynthesis of Phosphatidylethanolamine from ethanolamine.

Explanation of Solution

Phosphatidylethanolamine is generally the second most abundant phospholipid present in animal and plant lipids. It is usually the main component of lipid in the microbial membranes.

One of the main pathways for the biosynthesis of phosphatidylethanolamine is the CDP-ethanolamine pathway. In this pathway phospholipid biosynthesis, de novo in animals and plants.

Phosphatidylethanolamine biosynthesis takes place in the cytosol. Phosphorylation of ethanolamine occurs by two definite enzymes: Ethanolamine kinases and cytidine triphosphate (CTP). Then there is a reaction for the formation of diacylglycerol from glycerol and 2 fatty acids. This diacylglycerol then form phosphatidylethanolamine. The whole reaction involves the consumption of 3 ATP molecules.

The balance reaction equation involving the ethanolamine, glycerol and fatty acids in the formation of phosphatidylethanolamine is as follows:

$\begin{array}{l}\text{Glycerol + }\text{3 ATP + }{\text{2 fatty acids + 2 H}}_{\text{2}}\text{O }\text{+ CTP }\text{+ }\text{ethanolamine}\to \\ \text{Phosphatidylethanolamine }\text{+ }\text{CMP + ADP + 2 AMP + }\text{6 Pi}{\text{ + 3 H}}^{\text{+}}\end{array}$

So, CDP-ethanolamine is the activated reactant in the biosynthesis of Phosphatidylethanolamine from ethanolamine.

Interpretation Introduction

(c)

Interpretation:

The activated reactant in the biosynthesis of Ceramide from sphingosine should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

Acyl CoA is the activated reactant in the biosynthesis of Ceramide from sphingosine.

Explanation of Solution

Sphingolipids are a type of lipid molecules. The basic structural units of all sphingolipids are ceramides. Ceramides are a class of waxy lipid molecules. These are composed of sphingosine and a fatty acid. These are present in high concentrations inside the eukaryotic cell membrane.

Usually, an organic aliphatic amino alcohol sphingosine is present in these molecules. A ceramide is formed by the reconversion of sphingosine. This reconversion takes placewith the help of the condensation process. It takes place with a fatty-acyl-CoA, that is catalyzed by various Ceramide synthase (CerS) enzymes.

So, Acyl CoA is the activated reactant in the biosynthesis of Ceramide from sphingosine.

Interpretation Introduction

(d)

Interpretation:

The activated reactant in the biosynthesis of Sphingomyelin from Ceramide should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

Phosphatidylcholine is the activated reactant in the biosynthesis of Sphingomyelin from Ceramide.

Explanation of Solution

Sphingomyelin synthase or SMS is a class of enzymes which produces Sphingomyelin (SM) by the transfer of a phosphocholine moiety on a ceramide. Phosphatidylcholine or PC is believed to be the phosphocholine donor of the reaction which resultsin the production of diacylglycerol (DAG). It is a significant bioactive lipid.In sphingomyelin, the end unit of the hydroxyl group of a ceramide gets changed with a phosphorylcholine.

So, Phosphatidylcholine is the activated reactant in the biosynthesis of Sphingomyelin from Ceramide.

Interpretation Introduction

(e)

Interpretation:

The activated reactant in the biosynthesis of Cerebroside from Ceramide should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

UDP-galactose or UDP-glucose is the activated reactant in the biosynthesis of Cerebroside from Ceramide.

Explanation of Solution

In the formation of phosphoglyceride, an activated UDP-glucose is produced. The activated intermediate undergoes reaction with a hydroxyl group such as side chain of serine, diacylglycerol, the terminus of glycogen. In a cerebroside, the hydroxyl group of ceramides is linked with a glucose or galactose unit.

Therefore, during the biosynthesis of mono-glycosyl ceramides needs the direct transfer of carbohydrate moiety from a sugar-nucleotide. These sugar nucleotides include UDP-glucose or uridine5-diphosphate (UDP)-galactose to the ceramide unit.

So, UDP-galactose or UDP-glucose is the activated reactant in the biosynthesis of Cerebroside from Ceramide.

Interpretation Introduction

(f)

Interpretation:

The activated reactant in the biosynthesis of ganglioside from G_M1 ganglioside G_M2 ganglioside should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

UDP-galactose is the activated reactant in the biosynthesis of ganglioside from G_M1 ganglioside G_M2 ganglioside.

Explanation of Solution

Gangliosideis a group of complex lipids. These are present in the grey matter of the human brain. These sugar nucleotides include UDP-glucose or uridine 5-diphosphate (UDP)-galactose to the ceramide unit. As a ganglioside contain the attachment of an oligosaccharide to the hydroxyl group of ceramides. Uridine 5-diphosphate (UDP)-galactose helps in the biosynthesis of ganglioside from G_M1 ganglioside G_M2 ganglioside.

So, UDP-galactose is the activated reactant in the biosynthesis of ganglioside from G_M1 ganglioside G_M2 ganglioside.

Interpretation Introduction

(g)

Interpretation:

The activated reactant in the biosynthesis of farnesyl pyrophosphate from geranyl pyrophosphate should be determined.

Concept introduction:

Phospholipids are the basic constituent of all cell membranes. They usually form lipid bilayers. The structure of a phospholipid molecule has a tail made up of two hydrophobic fatty acid and a hydrophilic head comprising of a phosphate group.

Answer to Problem 13P

Geranyl pyrophosphate is the activated reactant in the biosynthesis of farnesyl pyrophosphate from geranyl pyrophosphate.

Explanation of Solution

Protein prenylation can be defined as a post-translational modification that involves the linking of a 15 or 20 carbon isoprenoid group to a residue of the protein. Isoprenoid units such as Farnesyl (C₁₅) is comprised of three repeating isoprene subunits. Whereas the geranylgeranyl (C₂₀) is comprised of four such subunits.

Geranyl pyrophosphate (or GPP) is also known by another name geranyl diphosphate (or GDP). It acts as an intermediate of the HMG-CoA reductase pathway. It is generally useful in the biosynthesis of farnesyl pyrophosphate, geranylgeranyl pyrophosphate.

So, Geranyl pyrophosphate is the activated reactant in the biosynthesis of farnesyl pyrophosphate from geranyl pyrophosphate.