BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 15. THE PENTOSE PHOSPHATE PATHWAY AND GLUCONEOGENESIS

15.20. Substrate Cycles Amplify Metabolic Signals and Heat Production

A pair of reactions, such as the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate and the reverse Hydrolysis of the latter back to fructose-6-phosphate, is called a substrate cycle. As previously mentioned, in most Cells these reactions never proceed simultaneously at their maximal possible rates owing to reciprocal allosteric control. However, isotope-labeling studies have shown that phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate does take place during Gluconeogenesis. The limited operation of such cycles has also been detected in the case of other pairs of oppositely directed irreversible reactions. The existence of these cycles was initially attributed to imperfect Metabolic Regulation, and Substrate Cycles were sometimes referred to as futile, or idle, cycles. Today, however, it appears much more likely that substrate cycles serve a definite biological purpose. One possibility is that these cycles amplify biological signals. Suppose that The rate of conversion of A to B is 100, and of B to A is 90, so that the initial net output of the reaction is 10. Let us assume that an allosteric effector increases the rate of the A → B reaction by 20% (to 120) and reciprocally decreases the rate of the B → A reaction by 20% (to 72). The new net output is 48. Thus, a 20% change in the rates of oppositely directed reactions leads to a 480% increase in the net output of the process. In the example shown in Fig. 15.7, this Amplification is achieved through the hydrolysis of ATP.

Class="center">Fig. 15.7. An example of an ATP-stimulated substrate cycle operating at two different rates. A small change in the rates of two oppositely directed reactions leads to a significant change in the net yield of product B

Another potential Biological Role of substrate cycles is to generate heat produced by the hydrolysis of ATP. A striking example of this phenomenon is found in bumblebees, which must maintain a thoracic Temperature of about 30°C in order to fly. Bumblebees are able to maintain such a high thoracic temperature and forage even at an ambient temperature of only 10°C because their flight Muscle possesses high activity of both Phosphofructokinase and fructose bisphosphatase. Since this fructose bisphosphatase is not inhibited by AMP, there is reason to believe that this enzyme is specifically designed for heat generation. By contrast, in the flight muscle of the honeybee, fructose bisphosphatase activity is almost entirely absent, and accordingly, the honeybee cannot fly at low ambient temperatures. An excessively high rate of the fructose-6-phosphate ⇌ fructose-1,6-bisphosphate cycle can cause excessive heat production. A condition known as malignant hyperthermia is triggered by the anesthetic halothane in susceptible strains of pigs.

15.21. Lactate Formed by Contracting Muscle Is Converted into Glucose in the Liver

The primary raw material for gluconeogenesis is lactate produced by active Skeletal Muscle. In contracting skeletal muscle under anaerobic conditions, the rate of Pyruvate formation through Glycolysis exceeds the rate of its oxidation in The Tricarboxylic Acid Cycle. Furthermore, the rate of NADH production in glycolysis is higher than the rate of its oxidation in the Respiratory Chain. The continuation of glycolysis depends on the availability of NAD⁺ for The oxidation of glyceraldehyde-3-phosphate, and the generation of NAD⁺ is carried out by Lactate dehydrogenase, which reduces pyruvate to lactate while oxidizing NADH to NAD⁺.

Lactate is a metabolic dead end. For further metabolic transformations, it must first be converted back into pyruvate.

The sole purpose of reducing pyruvate to lactate is to regenerate the NAD⁺ required for glycolysis to proceed in active skeletal muscle. Lactate production takes time and shifts part of the metabolic burden from the Muscles to the Liver.

The Plasma Membranes of most cells are highly permeable to lactate and pyruvate. Both compounds diffuse out of active skeletal muscle into the Blood and are transported to the liver. Significantly more lactate than pyruvate is transported because of the high [NADH]/[NAD⁺] ratio in contracting skeletal muscle. Lactate entering the liver is oxidized to pyruvate, a process favored by the low [NADH]/[NAD⁺] ratio in the hepatic Cytosol. Pyruvate is then converted into glucose in the liver via The gluconeogenesis pathway. Glucose is subsequently released into the blood and taken up by skeletal muscles. Thus, the liver supplies glucose to contracting muscles, which derive ATP from the glycolytic conversion of glucose to lactate. Next, glucose is synthesized from lactate in the liver. These transformations constitute the Cori cycle (Fig. 15.8).

Fig. 15.8. The Cori cycle. Lactate produced by active muscle is converted into glucose in the liver. This cycle transfers a portion of the metabolic "load" from the active muscle to the liver

[NADH]/[NAD⁺] ratio in contracting skeletal muscle. Lactate entering the liver is oxidized to pyruvate, a process favored by the low [NADH]/[NAD⁺] ratio in the hepatic cytosol. Pyruvate is then converted into glucose in the liver via the gluconeogenesis pathway. Glucose is subsequently released into the blood and taken up by skeletal muscles. Thus, the liver supplies glucose to contracting muscles, which derive ATP from the glycolytic conversion of glucose to lactate. Next, glucose is synthesized from lactate in the liver. These transformations constitute the Cori cycle (Fig. 15.8).

The processes described are facilitated by differences in the catalytic properties of muscle and liver lactate dehydrogenases. Lactate dehydrogenase is a tetramer composed of subunits with a Molecular Weight of 35 kDa. There are two types of polypeptide chains, designated M and H, which can form five types of tetramers: M₄, M₃H, M₂H₂, M₁H₃, and H₄. These forms are called Isoenzymes (or isozymes). The M₄ isoenzyme has a significantly higher affinity for pyruvate than the H₄ isoenzyme, while the other isoenzymes exhibit intermediate activities. Lactate dehydrogenase isoenzymes have been intensively studied, yet the reason for the existence of Multiple Forms of the enzyme remains an enigma.