Enzymes pyruvate carboxylase

Pyruvic acid is an intermediate in the fermentation of grains. During fermentation the enzyme pyruvate carboxylase causes the pyruvate ion to release carbon dioxide. In one experiment a 200.-mL aqueous solution of the pyruvate in a sealed, rigid 500.-mL flask at 293 K had an initial concentration of 3.23 mmol-L -l. Because the concentration of the enzyme was kept constant, the reaction was pseudo-first order in pyruvate ion. The elimination of CU2 by the reaction was monitored by measuring the partial pressure of the C02 gas. The pressure of the gas was found to rise from zero to 100. Pa in 522 s. What is the rate constant of the pseudo-first order reaction ... 

The free glucose produced by this reaction is supplied to the blood from the tissues. As exemplified by gluconeogenesis, one may easily envision the economical organization of these metabolic routes, since, apart from four special gluconeogenesis enzymes-pyruvate carboxylase, phosphopyruvate carboxylase, fructose bisphosphatase, and glucose 6-phosphatase-individual glycolytic enzymes are also used in the gluconeogenesis. 

The reactions that convert pyruvate to intermediates of the TCA cycle are called the anaplerotic reactions. Pyruvate, which can be made only from glucose or some of the amino acids, can be converted to oxaloacetate by the enzyme pyruvate carboxylase or to malate by malic enzyme. 

As we have seen, normally pyruvate would be the substrate for pyruvate dehydrogenase complex to form acetyl-CoA, but during fasting in the absence of glucose, acetyl -CoA for the TCA cycle is derived from fatty acid (3-oxidation (see Section 7.5.2) so pyruvate is diverted into oxaloacetate by the enzyme pyruvate carboxylase. Thus any amino acids whose carbon skeletons can be converted into pyruvate, OAA or another substrate of the TCA cycle, can be used for glucose synthesis. 

The theory of regulation of the cycle is as follows. First, an increase in oxaloacetate concentration increases the activity of citrate synthase and hence the cycle. The concentration of oxaloacetate is regulated by the activity of the enzyme pyruvate carboxylase, which catalyses the reaction ... 

Pyruvate is converted to oxaloacetate by carboxylation using the enzyme pyruvate carboxylase that is located in the mitochondrial matrix. 

From all the hypotheses mentioned above, the low flux of pyruvate into the TCA cycle is better explained by a decreased activity of several enzymes pyruvate carboxylase, pyruvate phosphoenol carboxykinase, pyruvate dehydrogenase, and malic enzyme II (Figure 4.3). Then, the pyruvate accumulated is converted to lactate by the enhanced catalytic action of the lactate dehydrogenase, as an alternative pathway to generate energy for cellular processes. 

Biotin, an essential water-soluble B-complex vitamin, is the coenzyme for four human carboxylases (Fig. 12-2) These include the three mitochondrial enzymes pyruvate carboxylase, which converts pyruvate to oxaloacetate and is the initial step of gluconeogenesis propionyl-CoA carboxylase, which catabolizes several branched-chain amino acids and odd-chain fatty acids and 3-methylcrotonyl-CoA carboxylase, which is involved in the catabolism of leucine and the principally cytosolic enzyme, acetyl-CoA carboxylase, which is responsible for the... 

The enzyme pyruvate carboxylase (Chap. 12) from chicken liver is an oligomer composed of four identical subunits. The enzyme loses its catalytic activity when cooled below 277 K. Assuming that this loss of activity reflects dissociation of the tetrameric enzyme into its subunits, what can you deduce about the relative importance of enthalpic and entropic effects in the association of the subunits in the tetrameric enzyme ... 

Step A, the conversion of pyruvate to phosphoenolpyruvate, is accomplished by a circuitous process commencing with pyruvate entering the mitochondrion, which for gluconeogenesis to occur must be in a high-energy state. Under these conditions, the mitochondrial enzyme pyruvate carboxylase catalyzes the conversion of pyruvate to oxaloacetate-. 

How is oxaloacetate replenished Mammals lack the enzymes for the net conversion of acetyl CoA into oxaloacetate or any other citric acid cycle intermediate. Rather, oxaloacetate is formed by the carboxylation of pyruvate, in a reaction catalyzed by the biotin-dependent enzyme pyruvate carboxylase. 

The chicken liver enzyme pyruvate carboxylase also has a natural requirement for Mn. The distances between Mn and the carboxyl and carbonyl carbon atoms of pyruvate (8) in the enzyme-pyruvate complex... 

Like many C02-fixing enzymes, pyruvate carboxylase contains biotin bound through the e-NH2 of a lysyl residue (Chapter 18). 

Synthesis of PEP. PEP synthesis from pyruvate requires two enzymes pyruvate carboxylase and PEP carboxykinase. Pyruvate carboxylase, found within mitochondria, converts pyruvate to oxaloacetate (OAA) ... 

Figure 11.9 (a) Substrate-enzyme bipartite graph where one set of nodes, E.C.6.4.1.1 and E.C.2.3.3.1, represents the enzymes pyruvate carboxylase and citrate (Si)-synthaseand the other set represents the substrates and products, (b) Substrate graph, (c) Enzyme-centric graph, (d) Reaction-centric graph. (Courtesy of Rahman and Schomburg, 2006.)... 

The significance of manganese as a cofactor for the enzyme pyruvate carboxylase is widely assumed. It may also act as a nonspecific activator for some other enzymes [14],... 

A further example is the enzyme pyruvate carboxylase which is involved both in gluooneo-genesis and in maintaining the level of citrate cycle intermediates and requires acetyl-CoA as an obligatory activator. 

Sugar fermentation follows the same path as muscle glycolysis in that a chain of intermediate phosphate esters is formed but when the pyruvate stage is reached, the enzyme pyruvic carboxylase, which is present in yeast but not in muscle, decarbosylates the pyruvic acid to acetaldehyde and carbon dioxide, a characteristic end-product of the fermentation. 

Lactate and alanine enter as pyruvate following the activities of lactate dehydrogenase (Figure 11.4) and alanine aminotransferase (Section 16.3). The first of the bypass reactions, the objective of which is to overcome the unfavourable energetics of a reversal of the pyruvate kinase reaction, seems a tortuous route (Figure 11.11). The reaction sequence relies on two important enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Since pyruvate carboxylase is located exclusively in the mitochondrion, pyruvate must cross the inner mitochondrial membrane (Section 12.2). Oxaloacetate produced by pyruvate carboxylase cannot traverse the inner membrane and is reduced by malate dehydrogenase into l-malate. This step is the reversal of the tricarboxylate cycle reaction (Section 12.4). Malate may, of... 

In all aerobic organisms, FA biosynthesis occurs via two different metabolic pathways (1) tricarboxylic acid cycle (TCA) or Krebs cycle and (2) reductive carboxylation pathway. Albert Szent-Gyorgyi (1893-1986, Hxmgary) discovered FA catalysis during his study (including on vitamin C) on cellular combustion process (TCA cycle) for which he was awarded the Noble Prize in Physiology or Medicine in 1937 (www.nobelprize.org). TCA involves CO2 fixation coupled with the conversion of pyruvate to oxaloacetate, the precursor to malate and fuma-rate (Fig. 8.3). Reductive CO2 fixation catalyzed by the enzyme pyruvate carboxylase xmder... 

It was also beheved that the gluconeogenic machinery, including four critical enzymes pyruvate carboxylase, PEP carboxykinase, fructose-1,6-diphosphatase, and glucose-6-phosphatase could be fuUy expressed exclusively in the liver and in the kidney. Specifically, with the exception of hepatocytes and renal cortical cells, it was considered impossible that any glucose synthesized de novo or released Irom local glycogen stores could be released in the systemic circulation, because of the absence of gJucose-6-phosphatase. 

The carbon used for fatty-acid synthesis typically derives from the products of glycolysis. The end product of glycolysis, pyruvate, enters the mitochondria and becomes the substrate for two separate reactions. In one, pyruvate is decarboxylated via the pyruvate dehydrogenase complex, yielding acetyl-CoA. Lipogenic tissues also contain another mitochondrial enzyme, pyruvate carboxylase, which converts pyruvate to the four-carbon acid oxaloace-tate (OAA). Acetyl-CoA and oxaloacetate condense to form the six-carbon acid citrate. As citrate accumulates within the mitochondrion, it is exported to the cytoplasm, where it is converted back to oxaloacetate and acetyl-CoA. Cytoplasmic acetyl-CoA is the fundamental building block for de novo synthesis of fatty acids. 

Figure 6 Fatty-acid biosynthesis. Cytoplasmic acetyl-CoA (AcCoA) is the primary substrate for de novo fatty-acid synthesis. This two-carbon compound most commonly derives from the glycolytic degradation of glucose, and its formation is dependent upon several reactions in the mitochondria. The mitochondrial enzyme pyruvate carboxylase is found primarily in tissues that can synthesize fatty acids. AcCoA is converted to maionyl-CoA (MalCoA) by acetyl-CoA carboxylase. Using AcCoA as a primer, the fatty-acid synthase multienzyme complex carries out a series of reactions that elongate the growing fatty acid by two carbon atoms. In this process MalCoA condenses with AcCoA, yielding an enzyme-bound four-carbon /3-ketoacid that is reduced, dehydrated, and reduced again. The product is enzyme-bound 4 0. This process is repeated six more times, after which 16 0 is released from the complex. The reductive steps require NADPH, which is derived from enzyme reactions and pathways shown in grey. Enz refers to the fatty acid synthase multienzyme complex.

---