Question

Using thermodynamic terms, describe the mechanism by which UDP-glucose is formed.

NADPH-reducing equivalents important for the biosynthesis of fatty acids come from several sources. Explain.

What is the role of CTP in the synthesis of a phospholipid?

What are the three stages of cholesterol synthesis?

Answer 1

1.

Glucose-1-phosphate + UTP----> UDP glucose + PPi

It isa hydrolysis reaction where H20 hydrolyses pyrophosphate to two Inorganic phosphate molecules(2Pi).

This hydrolysis reaction is similar to that of ATP. The overall reaction yields 30.5 kI/mol. The coupling of these
two reactions enbales the endergonic reaction (UDP formation) to occur easily.

2. Oxaloacetate is used to generate NADPH for biosynthesis in a two‐step sequence. The first step is the malate dehydrogenase catalyzing malate substrate forming pyruvate, CO2 and NADPH where NAD⁺  is oxidised by malate dehydrogenease into NADPH

NADPH is also formed by the pentose phosphate pathway where glucose is convereted into ribose that is be used in synthesis of nucleotides and nucleic acids, or it can also be utilized to form pyruvate

3.

(a) CTP is an essential nucleotide synthesized from UTP by the enzyme CTP synthetase.

(b) It serves to activate phosphatidate which serves as an intermediate involved in phospholipid synthesis.

(c) In the de novo synthesis, the CTP is used to activate the phosphatidate to form CDP-diacylglycerol and pyrophosphate. The reaction is energised by the coupled reaction of hydrolysis of the PPi to 2 Pi. The activated diacylglycerol reacts with alcohol called inositol leading to formation of cytidine monophosphate (CMP)

4.

(a) Acetyl Co.A gets converted into Isopoentyl pyrophosphate, thus acetyl CO.A serves as building block for cholesterol synthesis

(b) Six molecules of isopentyl pyrophosphate are combined to form a 30 carbon molecule called squalene

(c) Squalene undergoes cyclization to form alanosterol intermediate which gets converted into cholesterol.

3.