Glucose storage forms

Increase in glycogenesis (formation of glycogen, the storage form of glucose)... 

There are only three types of storage molecules glycogen, fat, and protein. These three storage forms are all connected, and that s what energy metabolism is all about—the connections. When metabolism works normally, we re assured of a relatively constant supply of energy and glucose. 

Fat provides a long-term storage form for energy (ATP). Fat provides zero glucose equivalents. 

Fat is only an energy storage form (Fig. 17-4). Fat cannot be converted to carbohydrate equivalents. This is a very important point. Remember it The reason for this is a bit subtle. The carbon skeleton of fatty acids is metabolized to acetyl-CoA only. Glucose precursors such as oxaloacetate can be synthesized from acetyl-CoA by going around the TCA cycle. However, acetyl-CoA has 2 carbon atoms. Going around the TCA cycle burns off 2 carbon atoms (as C02). The net number of carbon atoms that ends up in oxaloacetate is then zero. No carbohydrate can be made from fat.5... 

Glucose (or more correctly its metabolites) is essential for the functioning of the TCA cycle. For the TCA cycle to keep turning, the intermediates of the cycle must be maintained at a reasonable level. Since these intermediates are used for things other than the TCA cycle, they must be replaced constantly. The trouble is that the intermediates of the TCA cycle cannot be synthesized from fat (our most abundant storage form of energy). So we re got to have glucose or its equivalent to bum fat. 

A. Glycogen is the storage form of glucose mainly found in liver and muscle. 

Glycogen Storage form of glucose in animals and humans. 

The catalytic activity of enzymes is controlled in several ways. Reversible allosteric control is especially important. For example, the first reaction in many biosynthetic pathways is allosterically inhibited by the ultimate product of the pathway. The inhibition of aspartate trans carbamoyl as e by cytidine triphosphate (Section 10.1) is a well-understood example offeedback inhibition. This type of control can be almost instantaneous. Another recurring mechanism is reversible covalent modification. For example, glycogen phosphorylase, the enzyme catalyzing the breakdovm of glycogen, a storage form of sugar, is activated by phosphorylation of a particular serine residue when glucose is scarce (Section 21.2.1). 

Acetyl CoA is the fuel for the citric acid cycle. This important molecule is formed from the breakdown of glycogen (the storage form of glucose), fats, and many amino acids. Indeed, as we will see in Chapter 22. fats contain strings of reduced two-carbon units that are first oxidized to acetyl CoA and then completely oxidized to CO2 by the citric acid cycle. 

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