Glycolysis:- An Overview: Glycolysis Has Two Phases
The breakdown of the six-carbon glucose into two molecules of the three-carbon pyruvate occurs in ten steps, the first five of which constitute the preparatory phase (Fig. 14–2a). In these reactions, glucose is first phosphorylated at the hydroxyl group on C-6 (step 1). The D-glucose 6-phosphate thus formed is converted to D fructose 6-phosphate (step 2), which is again phosphorylated, this time at C-1, to yield D-fructose 1,6 bisphosphate (step 3). For both phosphorylations, ATP is the phosphoryl group donor. As all sugar derivatives in glycolysis are the D isomers, we will usually omit the D designation except when emphasizing stereochemistry. Fructose 1,6-bisphosphate is split to yield two three-carbon molecules, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate (step 4); this is the “lysis” step that gives the pathway its name. The dihydroxyacetone phosphate is isomerized to a second molecule of glyceraldehyde 3-phosphate (step 5), ending the first phase of glycolysis. From a chemical perspective, the isomerization in step 2 is critical for setting up the phosphorylation and C-C bond cleavage reactions in steps 3 and 4, as detailed later. Note that two molecules of ATP are invested before the cleavage of glucose into two three-carbon pieces; later there will be a good return on this investment. To summarize: in the preparatory phase of glycolysis the energy of ATP is invested, raising the free-energy content of the inter mediates, and the carbon chains of all the metabolized hexoses are converted into a common product, glyceraldehyde 3-phosphate. The energy gain comes in the payoff phase of glycolysis (Fig. 14–2b). Each molecule of glyceraldehyde 3-phosphate is oxidized and phosphorylated by inorganic phosphate (not by ATP) to form 1,3-bisphospho glycerate (step 6). Energy is then released as the two molecules of 1,3-bisphosphoglycerate are converted to two molecules of pyruvate (steps 7 through 10). Much of this energy is conserved by the coupled phosphorylation of four molecules of ADP to ATP. The net yield is two molecules of ATP per molecule of glucose used, because two molecules of ATP were invested in the preparatory phase. Energy is also conserved in the payoff phase in the formation of two molecules of NADH per molecule of glucose. In the sequential reactions of glycolysis, three types of chemical transformations are particularly noteworthy: (1) degradation of the carbon skeleton of glucose to yield pyruvate, (2) phosphorylation of ADP to ATP by high-energy phosphate compounds formed during glycolysis, and (3) transfer of a hydride ion to NAD+, forming NADH.
Fates of Pyruvate With the exception of some interesting variations in the bacterial realm, the pyruvate formed by glycolysis is further metabolized via one of three catabolic routes. In aerobic organisms or tissues, under aerobic conditions, glycolysis is only the first stage in the complete degradation of glucose (Fig. 14–3). Pyruvate is oxidized, with loss of its carboxyl group as CO2, to yield the acetyl group of acetyl-coenzyme A; the acetyl group is then oxidized completely to CO2 by the citric acid cycle. The electrons from these oxidations are passed to O2 through a chain of carriers in the mitochondrion, to form H2O. The energy from the electron-transfer reactions drives the synthesis of ATP in the mitochondrion.
The second route for pyruvate is its reduction to lactate via lactic acid fermentation. When vigorously contracting skeletal muscle must function under low oxygen conditions (hypoxia), NADH cannot be reoxidized to NAD+, but NAD+ is required as an electron acceptor for the further oxidation of pyruvate. Under these conditions pyruvate is reduced to lactate, accepting electrons from NADH and thereby regenerating the NAD+ necessary for glycolysis to continue. Certain tis sues and cell types (retina and erythrocytes, for example) convert glucose to lactate even under aerobic conditions, and lactate is also the product of glycolysis under anaerobic conditions in some microorganisms (Fig. 14–3). The third major route of pyruvate catabolism leads to ethanol. In some plant tissues and in certain invertebrates, protists, and microorganisms such as brewer’s yeast, pyruvate is converted under hypoxic or anaerobic conditions into ethanol and CO2, a process called ethanol (alcohol) fermentation (Fig. 14–3). The oxidation of pyruvate is an important catabolic process, but pyruvate has anabolic fates as well. It can, for example, provide the carbon skeleton for the syn thesis of the amino acid alanine. We return to these anabolic reactions of pyruvate in later chapters.
ATP Formation Coupled to Glycolysis During glycolysis some of the energy of the glucose molecule is conserved in ATP, while much remains in the product, pyruvate. The overall equation for glycolysis is
Glucose+2NAD+ +2ADP+2Pi → 2 pyruvate 2NADH+2H++2ATP + 2H2O (14–1)
For each molecule of glucose degraded to pyruvate, two molecules of ATP are generated from ADP and Pi. We can now resolve the equation of glycolysis into two processes—the conversion of glucose to pyruvate, which is exergonic:
Glucose + 2NAD+→2 pyruvate+2NADH+2H+ (14–2) ΔG01=-146 kJ/mol
FIGURE 14–2 The two phases of glycolysis. For each molecule of glucose that passes through the preparatory phase (a), two molecules of glyceraldehyde 3-phosphate are formed; both pass through the payoff phase (b). Pyruvate is the end product of the second phase of glycolysis. For each glucose molecule, two ATP are consumed in the preparatory phase and four ATP are produced in the payoff phase, giving a net yield of two ATP per molecule of glucose converted to pyruvate. The numbered reaction steps are catalyzed by the enzymes listed on the right, and also correspond to the numbered headings in the text discussion. Keep in mind that each phosphoryl group, represented here as P, has two negative charges (-PO3-2).
and the formation of ATP from ADP and Pi, which is endergonic:
2ADP+2Pi →2ATP+2H2O (14–3)
ΔG2=2(30.5 kJ/mol) 61.0 kJ/mol
The sum of Equations 14–2 and 14–3 gives the overall standard free-energy change of glycolysis, ΔG0s:
ΔG0s= ΔG01 + ΔG02 =-146 kJ/mol+61.0 kJ/mol=- 85 kJ/mol
Under standard conditions and in the cell, glycolysis is an essentially irreversible process, driven to completion by a large net decrease in free energy. At the actual in tracellular concentrations of ATP, ADP, and Pi (see Box 13–1) and of glucose and pyruvate, the energy released in glycolysis (with pyruvate as the end product) is recovered as ATP with an efficiency of more than 60%. Energy Remaining in Pyruvate Glycolysis releases only a small fraction of the total available energy of the glucose molecule; the two molecules of pyruvate formed by glycolysis still contain most of the chemical potential energy of glucose, energy that can be extracted by oxidative reactions in the citric acid cycle and oxidative phosphorylation. Importance of Phosphorylated Intermediates Each of the nine glycolytic intermediates between glucose and pyruvate is phosphorylated (Fig. 14–2). The phosphoryl groups appear to have three functions.
1. Because the plasma membrane generally lacks transporters for phosphorylated sugars, the phosphorylated glycolytic intermediates cannot leave the cell. After the initial phosphorylation, no fur ther energy is necessary to retain phosphorylated intermediates in the cell, despite the large difference in their intracellular and extracellular concentrations. 2. Phosphoryl groups are essential components in the enzymatic conservation of metabolic energy. Energy released in the breakage of phosphoanhydride bonds (such as those in ATP) is partially conserved in the formation of phosphate esters such as glucose 6-phosphate. High-energy phosphate compounds formed in glycolysis (1,3-bisphos phoglycerate and phosphoenolpyruvate) donate phosphoryl groups to ADP to form ATP. 3. Binding energy resulting from the binding of phosphate groups to the active sites of enzymes lowers the activation energy and increases the specificity of the enzymatic reactions (Chapter 6). The phosphate groups of ADP, ATP, and the glycolytic intermediates form complexes with Mg2+, and the substrate binding sites of many glycolytic enzymes are specific for these Mg2+ complexes. Most glycolytic enzymes require Mg2+ for activity.
FIGURE 14–3 Three possible catabolic fates of the pyruvate formed in glycolysis. Pyruvate also serves as a precursor in many anabolic re actions, not shown here.
