Question

1. What is the function of the hexose monophosphate shunt and indicate in which cells each of the oxidative and non oxidative pathways play a greater role and why (5 points).

Answer 1

The hexose monophosphate shunt, also known as the pentose phosphate pathway, is a unique pathway parallel to glycolysis used to create products essential in the body. The pentose phosphate pathway takes place in the cytosol of the cell, the same location as glycolysis. The two most important products from this process are the ribose-5-phosphate sugar used to make DNA and RNA, and the NADPH molecules which help with building other molecules.

Similar to some of the processes in cellular respiration, the molecules that go through the pentose phosphate pathway are mostly made of carbon. The breakdown of the simple sugar, glucose, in glycolysis provides the first 6-carbon molecule required for the pentose phosphate pathway. During the first step of glycolysis, glucose is transformed by the addition of a phosphate group, generating glucose-6-phosphate, another 6-carbon molecule. The pentose phosphate pathway can use any available molecules of glucose-6-phosphate, whether they are produced by glycolysis or other methods.

There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars.

In oxidative phase, two molecules of NADP+ are reduced to NADPH, utilizing the energy from the conversion of glucose-6-phosphate into ribulose 5-phosphate. NADPH that is required for reductive biosynthetic reactions such as those of cholesterol biosynthesis, bile acid synthesis, steroid hormone biosynthesis, and fatty acid synthesis.

In the nonoxidative phase, the pathway catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of nonoxidative reactions that can result in the synthesis of five-carbon sugars for nucleotide biosynthesis or the conversion of excess five-carbon sugars into intermediates of the glycolytic pathway. All these reactions take place in the cytosol.

Hexose monophosphate shunt activity is also high in mature erythrocytes, the lens and cornea, all of which need NADPH for reduced glutathione production .