Tissue reserves of metabolic fuels

In the fed state, as well as providing for immediate energy needs, substrates are converted into storage compounds for use in the fasting state. There are two main stores of metabolic fuels  

In addition, there is an increase in the synthesis of tissue proteins after a meal, as a result of the increased availability of metabolic fuel to provide ATP for protein synthesis 

In the fasting state, which is the normal state between meals, these reserves are mobilized and used. Glycogen is a source of glucose, while adipose tissue provides both fatty acids and glycerol from triacylglycerol. Some of the relatively labile protein laid down in response to meals is also mobilized in fasting, and the amino acids are used both as a metabolic fuel and, more importantly, a source of citric acid cycle intermediates for gluconeogenesis. 

Fatty acids are synthesized by the successive addition of two-carbon units from acetyl CoA, followed by reduction. Like P-oxidation, fatty acid synthesis is a spiral sequence of reactions, with different enzymes catalysing the reaction sequence for synthesis of short, medium- and long-chain fatty acids. 

The malonyl group is transferred onto an acyl carrier protein, and then reacts with the growing fatty acid chain, bound to the central acyl carrier protein of the fatty acid synthase complex. The carbon dioxide that was added to form malonyl CoA is lost in this reaction. For the first cycle of reactions, the central acyl carrier protein carries an acetyl group, and the product of reaction with malonyl CoA is acetoacetyl-ACP in subsequent reaction cycles, it is the growing fatty acid chain that occupies the central ACP, and the product of reaction with malonyl CoA is a ketoacyl-ACP. 

---