Lipids

Relatively large biological molecule that mostly does not dissolve in water.Lipids include fats, oils, cholesterol, a number of steroid hormones, the major constituents of cell membranes, etc.Lipids are predominantly hydrophobic substances, though can have hydrophilic portions as well. An example of the latter are lipids associated with membranes, that is, lipid bilayers, include phospholipids and the molecule, cholesterol.Unlike proteins, carbohydrates, and nucleic acids, lipids are less homogenous in terms of their structures. In particular, lipids do not have a consistent subunit from which they are polymerized but instead are categorized together as a group based upon their relative inability to dissolve in aqueous solutions. That is, they are oil-like in the generic sense of that term. The video placed a whole new spin on the age of rap and showed that is just not for young persons.Furthermore, while being enjoyable it also teaches one about lipids in a fun interactive session.

Reference

“Michael Eskin Lipids Rap,” Youtube Video, 2:52,posted by”Lipids Get a Real Bad Rap: It’s Just Not Fair,” May 17,2012, http://www.youtube.com/watch?v=6lrG65DdBl8

The Citric Acid Cycle Is Controlled at Several Points

The rate of the citric acid cycle is precisely adjusted to meet an animal cell’s needs for ATP.The primary control points are the allosteric enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase

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Control of the Citric Acid Cycle. The citric acid cycle is regulated primarily by the concentration of ATP and NADH. The key control points are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

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Isocitrate dehydrogenase is allosterically stimulated by ADP, which enhances the enzyme’s affinity for substrates. The binding of isocitrate, NAD+, Mg2+, and ADP is mutually cooperative. In contrast, NADH inhibits iso-citrate dehydrogenase by directly displacing NAD+. ATP, too, is inhibitory. It is important to note that several steps in the cycle require NAD+ or FAD, which are abundant only when the energy charge is low.

A second control site in the citric acid cycle is α-ketoglutarate dehydrogenase. Some aspects of this enzyme’s control are like those of the pyruvate dehydrogenase complex, as might be expected from the homology of the two enzymes. α-Ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH, the products of the reaction that it catalyzes. In addition, α-ketoglutarate dehydrogenase is inhibited by a high energy charge. Thus, the rate of the cycle is reduced when the cell has a high level of ATP.

In many bacteria, the funneling of two-carbon fragments into the cycle also is controlled. The synthesis of citrate from oxaloacetate and acetyl CoA carbon units is an important control point in these organisms. ATP is an allosteric inhibitor of citrate synthase. The effect of ATP is to increase the value of KM for acetyl CoA. Thus, as the level of ATP increases, less of this enzyme is saturated with acetyl CoA and so less citrate is formed.

reference: “The Citric Acid cycle is Controlled at Several Points,” NCBI, assessed on April 10,  2013, http://www.ncbi.nlm.nih.gov/books/NBK22347/.

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