Krebs cycle, reductive

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The citric acid cycle, a nine-step process, also diverts chemical energy to the production of ATP and the reduction of NAD and FAD. In each step of the citric acid cycle (also known as the Krebs cycle) a glucose metabolite is oxidized while one of the carrier molecules, NAD or FAD, is reduced. Enzymes, nature s chemical catalysts, do a remarkable job of coupling the oxidation and reduction reactions so that energy is transferred with great efficiency. 

NADH, which enters the Krebs cycle. However, during cerebral ischaemia, metabolism becomes anaerobic, which results in a precipitous decrease in tissue pH to below 6.2 (Smith etal., 1986 Vonhanweh etal., 1986). Tissue acidosis can now promote iron-catalysed free-radical reactions via the decompartmentalization of protein-bound iron (Rehncrona etal., 1989). Superoxide anion radical also has the ability to increase the low molecular weight iron pool by releasing iron from ferritin reductively (Thomas etal., 1985). Low molecular weight iron species have been detected in the brain in response to cardiac arrest. The increase in iron coincided with an increase in malondialdehyde (MDA) and conjugated dienes during the recirculation period (Krause et al., 1985 Nayini et al., 1985). 

The surface metabolism hypothesis can also be used to deal with questions on the formation of the first cell structures for example, molecules required for the construction of cell membranes could have been formed via the so-called reductive Krebs cycle (citric acid cycle). 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

Fig. 7.6 A greatly simplified representation of the normal Krebs cycle (a) which occurs in today s living cells and the corresponding reductive process (b), which was possibly important in the metabolism of evolving systems...

G. Wachtershauser formulated his suggestions on the initial metabolic processes on the primeval Earth (as described above) in the form of six postulates and eleven theses (Wachtershauser, 1990b). Laboratory experiments planned to check, and perhaps confirm, these theories have already been attempted. The reductive Krebs cycle (rTCA cycle) has recently been the subject of much discussion. Smith and Morowitz consider that the rTCA cycle is a universal, possibly primordial, core process which could have provided substances for the synthesis of biomolecules on the young Earth (Smith and Morowitz, 2004). 

There are also voices critical of the rTCA cycle Davis S. Ross has studied kinetic and thermodynamic data and concludes that the reductive, enzyme-free Krebs cycle (in this case the sequence acetate-pyruvate-oxalacetate-malate) was not suitable as an important, basic reaction in the life evolution process. Data on the Pt-catalysed reduction of carbonyl groups by phosphinate show that the rate of the reaction from pyruvate to malate is much too low to be of importance for the rTCA cycle. In addition, the energy barrier for the formation of pyruvate from acetate is much too high (Ross, 2007). 

The situation is simpler for odd numbered fatty acyl derivatives as [3-oxidation proceeds normally until a 5-carbon unit remains, rather than the usual 4-carbon unit. The C5 moiety is cleaved to yield acetyl-CoA (C2) and propionyl-CoA (C3). Propionyl CoA can be converted to succinyl CoA and enter the TCA cycle so the entire molecule is utilized but with a slight reduction in ATP yield as the opportunity to generate two molecules of NADH by isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase is lost because succinyl-CoA occurs after these steps in the Krebs cycle (Figure 7.18). 

Diacids. The microbial generation of mahc, fumaric, and succinic acid essentially imphes Krebs cycle pathway engineering of biocatalytic organisms to overproduce oxaloacetate as the primary four-carbon diacid that subsequently undergoes reduction and dehydration processes (Scheme 2.9). The use of these four-carbon diacids as intermediate chemicals and the state of their desirable microbial production is briefly outlined. 

Fluoro amino acids have been incorporated into peptides, in order to ease the transport or reduce the systemic toxicity. Thus, trifluoroalanine, a powerful inhibitor of alanine racemase, is an essential enzyme for the biosynthesis of the cell wall of bacteria. It has a low antibiotic activity because of its very poor transport. In order to facilitate this transport, the amino acid has been incorporated into a peptide. This delivery allows a reduction of the doses, and thus the toxicity of the treatment is lowered.3-FIuorophenylaIanine (3-F-Phe) is a substrate of phenylalanine hydroxylase, which transforms it into 3-F-Tyr. 3-F-Tyr has a high toxicity for animals, due to its ultimate metabolization into fluorocitrate, a powerful inhibitor of the Krebs cycle (cf. Chapter 7). 3-F-Phe has a low toxicicity toward fungus cells, but when delivered as a tripeptide 3-F-Phe becomes an efficient inhibitor of the growth of Candida albicans. This tripeptide goes into the cell by means of the active transport system of peptides, where the peptidases set free the 3-F-Phe. ... 

The production of acetate from part of the incoming malate, which results in the formation of NADH, is balanced by the reduction of another portion of the malate to succinate. This reduction of malate to succinate occurs via two reactions that reverse part of the Krebs cycle (Fig. 20.1). In F. hepatica, succinate is not a... 

Most anaerobically functioning mitochondria use endogenously produced fumarate as a terminal electron-acceptor (see before) and thus contain a FRD as the final respiratory chain complex (Behm 1991). The reduction of fumarate is the reversal of succinate oxidation, a Krebs cycle reaction catalysed by succinate dehydrogenase (SDH), also known as complex II of the electron-transport chain (Fig. 5.3). The interconversion of succinate and fumarate is readily reversible by FRD and SDH complexes in vitro. However, under standard conditions in the cell, oxidation and reduction reactions preferentially occur when electrons are transferred to an acceptor with a higher standard redox potential therefore, electrons derived from the oxidation of succinate to fumarate (E° = + 30 mV) are transferred by SDH to ubiquinone,... 

As for chloroplast membranes, various compounds in mitochondrial membranes accept and donate electrons. These electrons originate from biochemical cycles in the cytosol as well as in the mitochondrial matrix (see Fig. 1-9) —most come from the tricarboxylic acid (Krebs) cycle, which leads to the oxidation of pyruvate and the reduction of NAD+ within mitochondria. Certain principal components for mitochondrial electron transfer and their midpoint redox potentials are indicated in Figure 6-8, in which the spontaneous electron flow to higher redox potentials is toward the bottom of the figure. As for photosynthetic electron flow, only a few types of compounds are involved in electron transfer in mitochondria—namely, pyridine nucleotides, flavoproteins, quinones, cytochromes, and the water-oxygen couple (plus some iron-plus-sulfur-containing centers or clusters). 

The most important functions of pantothenic acid are its incorporation in coenzyme A and acyl carrier protein (AGP). Both CoA and AGP/4-phosphopantetheine function metabolically as carriers of acyl groups. Coenzyme A forms high-eneigy thioester bonds with carboxylic acids. The most important coenzyme is acetyl CoA. Acetic acid is produced during the metabolism of fatty acids, amino acids, or carbohydrates. The active acetate group of acetyl CoA can enter the Krebs cycle and is used in the synthesis of fatty acids or cholesterol. AGP is a component of the fatty acid synthase multienzyme complex. This complex catalyzes several reactions of fatty acid synthesis (condensation and reduction). The nature of the fatty acid synthase complex varies considerably among different species (91). 

The Krebs cycle is a series of reactions catalyzed by sev en enzymes in mitochondria. Its function is to catalyze removal of electrons from nutrients and to transfer them to NAD and FAD, producing NADH plus H, and FADHj, respectively. These reduced cofactors exist only momentarily in their reduced (err oxidized) forms as they continually accept and then donate electrons to the respiratory chain. The respiratory chain, composed of a number of cytochromes, uses electrons for reduction of to water, This reduction process is accompanied by or coupled with the regeneration of ATP, that is, conversion of A DP back to ATE The overall effect may be summarized thus The Krebs cycle and respiratory chain arc used for oxidizing nutrients io COy and for the production of energy. 

Carbohydrates are introduced into the Krebs cycle at the point where pyruvate dehydrogenase catalyzes conversion of pyruvate to acetyl-Co A with the concomitant reduction of NAD. Citrate synthase catalyzes introduction of the 2-carbon unit of acetyl-CoA into the Krebs cycle. Pyruvate can arise from glucose, fructose, lactate, alanine, and glycerol. Acetyl-CcjAcan arise from pyruvate, as well as from fatty acids. Oxidation of fatty acids results in production of acetyl-Co A, which enters the Krebs cycle at the point catalyzed by citrate synthase. Breakdown of ketogenic amino adds also results in the production of acctyl-CoA, which enters the Krebs cycle at this point. Citrate and malate occur in high cor centrations in certain fruits and vegetables. These chemicals directly enter the Krebs cycle at the indicated points. 

Lactate dehydrogenase is closely associated with the final step in fermentative metabolism conversion of pyruvate to lactate- Citrate synthase is closely associated with oxidative metabolism, because this enzyme catalyzes the introduction of acetyl groups into the Krebs cycle. Continued operation of the Krebs cycle is dependent on the continued transfer of electrons from reduced NAD and FAD to the respiratory chain, which catalyzes the reduction of Oi to HjO. Table 4.14 lists lactate dehydrogenasc/citrate synthase activity ratios for muscles that contain a large proportion of white or red muscle fibers. The data demonstrate that the ratio is relatively high in white cell muscles but low in red cell muscles. 

---