|Introduction to Cell & Molecular Biology (BIOL121) - Dr. S.G. Saupe (email@example.com); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321|
Glucose Catabolism: Energy Releasing Pathways
Recall the general equation for the complete breakdown (catabolism) of glucose:
(CH2O)n + O2→ CO2 + H2O + energy (ATP = chemical energy, heat)
A. So, what can we learn/deduce from this equation:
Glucose catabolism is essentially the "reverse" of photosynthesis
Glucose catabolism is a redox reaction. Glucose (carbohydrate) is oxidized to carbon dioxide. The acceptor for the electrons is oxygen which is reduced to water.
The chemical bond energy of glucose is released as ATP and heat
This is the primary source of ATP for all aerobic organisms
This is an exergonic process
B. Hill Model Revisited - a diagram of the hill will be provided in class. Some take-home-lessons from the hilltop:
Anabolism (synthetic reactions) is analogous to pushing the rock uphill, catabolism (degradative reactions) is analogous to the rock rolling downhill;
Photosynthesis (an anabolic process) is analogous to pushing the rock uphill, respiration (a catabolic process) is analogous to the rock rolling downhill;
The energy required to push the rock (glucose) uphill comes from light (radiant energy);
The release of energy from glucose rolling downhill is coupled to ATP production (ca. 40% of the energy is trapped in ATP but more than half of the energy is lost as heat).
II. Rolling metabolic
rocks downhill - A look at Glucose Catabolism
Glucose catabolism occurs in a series of small, sequential, highly controlled and regulated steps (reactions). The processes involved are glycolysis, which is the first step of glucose breakdown, and it is followed by either fermentation or cellular respiration (depending on the availability of oxygen).
Why so many steps? Back to our hill model for an answer. There are two kinds of hills - those with a gradual, step-wise slope and those with a steep precipice or overhang. In each case the rock will roll down the hill and release the same total amount of energy, which is equal to the energy difference between the top and bottom of the hill (686 kcal/mol). However, the energy is released so quickly when rolling off a steep cliff that it is difficult to trap very much of it in a usable form (ATP). However, if the energy release occurs in a slow and orderly way, then it is possible to trap a greater percentage of energy as ATP.
III. Glycolysis - the first steps of glucose catabolism
Translation. Gr: glyco = sugar; Gr: lysis = split or cleave. Thus, the literal translation of glycolysis is the splitting of glucose.
Net reaction - glucose (C6, a sugar with 6 carbon atoms) is cleaved to yield two pyruvic acid molecules (C3).
Glycolysis consists of about 10 different reactions.
Each reaction is catalyzed by a different enzyme.
Organic acids, like pyruvic acid, are usually ionized at cellular pH values. We name the ionized form "ate". Thus, "pyruvic acid" and "pyruvate" refer to the same compound but in a slightly different form (ionized vs. not). Essentially these terms can be used synonymously.
Glycolysis occurs in the cytoplasm of virtually all cells. This suggests that glycolysis is an evolutionary ancient pathway. In fact, glycolysis likely evolved more than 3 billion years ago. Wow, it's even older than me!
Oxygen is not required. Glycolysis functions with (aerobic conditions) or without (anaerobic conditions) oxygen present.
Two ATP are required to get the "rock" rolling (hmmm, sounds like a beer brand). They "activate" the glucose making it energetically favorable to react. Remember how people who "misbehaved" were "drawn and quartered"? - this is the molecular version.
A total of four ATP are produced during the process at two different reactions. Thus, there is a net yield of 2 ATP.
ATP is produced during glycolysis by substrate level phosphorylation (vs. oxidative phosphorylation). For example: ADP + phosphoenolpyruvate (PEP)
There is one redox reaction during glycolysis. The oxidation of glucose begins during glycolysis. NAD+ accepts the electrons during the oxidation, and as a result it gets reduced. A total of 2 NADH are produced. Recall that NAD+ is a coenzyme (organic compound required by an enzyme for activity) that is used in redox reactions. Enzymes that catalyze redox reactions with the help of coenzymes such as NAD+ are called dehydrogenases.
Pyruvic acid is a branch point. It still has much energy and can be further degraded. If oxygen is present (aerobic conditions) pyruvate is metabolized via cellular respiration (i.e., citric acid cycle and ETC. Without oxygen, anaerobic conditions, pyruvate is metabolized via fermentation.
IV. Fermentation - stepping in an anaerobic environment
Occurs only in the absence of oxygen (anaerobic conditions)
Occurs in the cytoplasm
Occurs in most organisms
There are different types of fermentations depending upon the end products. The two common ones are:
Alcohol fermentations - yeast, plants. Important in baking and brewing industries. Don't over-water your plants because the soil pore spaces fill with water, the roots become anaerobic, fermentation begins, and the roots "pickle themselves".
Lactic acid fermentations - bacteria, humans. Commercially important in cultured dairy industry (yogurt, cheese), silage, pickles. In humans, excessive exercise results in periods of fermentation during which lactate builds up and contributes to muscle fatigue and soreness. What if humans had evolved alcohol fermentations instead of lactate ones? - every time you ran around the block you'd get drunk!
The ultimate function of fermentation is to regenerate
oxidized coenzymes, like NAD+.
Why is this necessary? - because glycolysis is the only source of ATP under anaerobic conditions. Recall that during the redox reaction in glycolysis, one of the key enzymes require OXIDIZED NAD+ as a coenzyme. After glycolysis works awhile, all of the coenzyme will be in the reduced form (NADH) and the reaction will stop because the enzyme won't have anywhere to dump off electrons. An analogy - imagine trying to bail out a boat full of water with a glass. The glass is like a coenzyme. It can be oxidized (empty) or full (reduced). You can continue to bail out the boat as long as the glass is empty. When full, it can't accept any more water. Thus, you're finished bailing, at least until you get another glass, or empty the one you have.
Fermentation vs. anaerobic respiration.
There is a difference! In a fermentation, electrons from coenzymes are donated back to part of the original substrate molecule. In a respiration, the electrons are donated to a substance, other than part of the original substrate. Thus, in an aerobic respiration, the electron acceptor is oxygen. In an anaerobic respiration, the acceptor is something else - like sulfur. This latter process occurs primarily in bacteria.
V. Aerobic or Cellular Respiration - stepping in air
This is not the same thing as breathing.
Occurs only in the presence of oxygen.
Occurs In the mitochondrion. Note, the entire cell of an aerobic bacteria can act like a mitochondrion to produce ATP aerobically). Remember the structure of a mitochondrion? (a) double membrane, inner and outer; (b) matrix - central or inner compartment; (c) inter-membrane space. Simply put, there is a liquid matrix where "biochemical reactions" occur, an inner membrane where electron transfer reactions occur and an area where low pH wonít do any harm (the "space")
B. Pyruvate Oxidation
Pyruvate from glycolysis is shuttled into the mitochondrion. As it enters, both of the pyruvate are oxidized (another redox reaction involving NAD+) and a carbon dioxide is lost from each. Note, that two carbons (one from each of the pyruvate molecules that originated from glucose) have been lost. The leftover two carbon piece is hooked onto a carrier (Acetyl coenzyme A).
C. Welcome to the Citric Acid Cycle or Krebís cycle
or Tricarboxylic Acid Cycle
It is named in honor of Sir Hans Krebs, who owned a bicycle shop in Kent, England. Just kidding - he was a British biochemist who worked out the details of many of the reactions. It is also called "Citric Acid Cycle" because citric acid is one of the important intermediate molecules or called the "Tricarboxylic Acid Cycle" (TCA) because citric acid has three carboxyl groups. Take your pick of names.
Occurs in the matrix of the mitochondrion (thus the reactants are water soluble)
There are several different reactions in the Citric Acid cycle, each catalyzed by a different enzyme.
There is one substrate level phosphorylation reaction that produces ATP. [Actually, the first product of the cycle is GTP (which is a nucleotide phosphate carrier like ATP). The GTP then donates a phosphate to ADP to make ATP.]
Carbon dioxide is released during two reactions (alpha-ketoglutarate dehydrogenase and isocitrate dehydrogenase) (or three total reactions if you include the reaction during which pyruvate was shuttled into the mitochondrion). Thus, during the Citric Acid cycle, the breakdown of glucose into carbon dioxide is completed.
There are four redox reactions, three of which yield reduced NADH and one FADH2. Thus, the oxidation of glucose is completed in the Kreb's cycle. If you count the redox reaction that occurred when shuttling pyruvate into the mitochondrion there is a total of 5 redox reactions in the mitochondrion.
D. Regenerating coenzymes in aerobic conditions
Recall that one problem of glycolysis was regenerating oxidized coenzymes. Under anaerobic conditions, this problem was solved by a variety of fermentation reactions or anaerobic respiration. Well, the solution is much more elegant in cells in an aerobic environment. Not only are oxidized coenzymes recovered, but the process is coupled to the production of ATP. Thus, the evolution of an oxygenated atmosphere allowed cells the added bonus of producing additional cellular energy while regenerating oxidized coenzymes. The electron transport chain (ETC) provided the means for this process.
E. Mitochondrial Electron Transport Chain
Occurs in the inner membrane (cristae). Thus, the components are more or less lipid-soluble and are embedded in the membrane
Occurs only under aerobic conditions
The mitochondrial ETC consists of a series of electron carriers that alternately accept and pass along an electron - like a hot potato or to use our glass analog - like a glass that is alternately filled (given an electron) and emptied (pass it off to the next carrier). In the process the carriers become reduced (glass full) and then oxidized again (glass empty).
There are four major groups (I, II, III, IV), called complexes, of electron carriers in the membrane. This is the reason why the text shows several blobs in the membrane. Each complex has a unique set of carriers. In addition, there are molecules that shuttle electrons from one complex to another. Among the carriers in the complexes (don't memorize these): Complex I - flavoproteins (FMN) and Fe-S proteins; Complex II - more flavoproteins, Fe-S proteins; Complex III - an assortment of proteins including cytochromes (c1 and b); Complex IV - cytochromes a,a3.
The sequence of electron flow occurs from complex I to
complex IV as depicted:
Since the complexes are physically separate in the
membrane, carriers must shuttle electrons from one complex to another.
Ubiquinone (coenzyme Q, or simply, Q) shuttles electrons from complex I to
complex III (and also between II and III). Cytochrome c shuttles electrons
from complex III to complex IV. Thus:
I → Q → III → cyt c → IV
NADH from the Kreb's cycle donates its electrons at the
end of the chain (to complex I). After donating its electrons, NADH gets
oxidized to NAD+. Thus, we can modify the reaction chain:
NADH → I → Q → III → cyt c → IV
The final acceptor of the electrons in the ETC is
oxygen. Oxygen becomes reduced to water. Thus:
NADH → I → Q → III → cyt c → O2
FADH2 and exogenous NADH (produced outside of the
mitochondria, i.e., during glycolysis) donate electrons to Complex II and
then on to oxygen. Thus, the electrons bypass Complex I. We can write this
FADH2/NADHex → II → Q → III → cyt c → IV → O2
The passage of electrons along the ETC is associated with the production of ATP.
F. Chemi-osmotic Production of ATP
The mechanism of ATP production is the same as for chloroplasts - via chemiosmosis.
A review of chemiosmosis: (a) at intervals along the ETC, protons (hydrogen ions) are moved from the matrix (or stroma) side of the membrane to the space between the inner and outer membranes; (b) the membrane is impermeable to protons which results in; (c) a pH and electrochemical (more positive charges in the intermembrane space than the matrix) gradient is established across the membrane. The pH of the matrix (or stoma) is alkaline (about pH 8) relative to the space (pH 5); (d) this gradient provides the driving force for ATP production; (e) ATP is made at an ATP synthase - which are liked stalked "lollipops", protein channels that span the membrane. Protons flow (like a waterwheel) through these.
There are three sites where protons are moved across the mitochondrial membrane "roughly" associated with complexes I, III and IV.
The total yield of ATP from oxidizing glucose is about 30 ATP/glucose; with most coming from the etc & chemi-osmosis (oxidative phosphorylation).
Note there is 15x more ATP produced under aerobic conditions (30) than under anaerobic conditions (2). Which do you choose?
V. Function of Aerobic Respiration/Glucose Catabolism
Energy production (30+ ATP/glucose)
Produce intermediates for many other metabolic reactions. Thus, various molecules can be siphoned off and used for other purposes.
Heat production. Glucose catabolism yields a TOTAL of approx. 38 ATP or 262 kcal (38 ATP x 7.3 kcal/mol ATP). Glucose has 686 kcal/mol. Thus, the efficiency of glucose metabolism is 262/686 x 100 = 38%. Or in other words, about 62% of the energy is lost as heat. This keeps us, shrews, and elephants warm. It also explains why compost piles generate lots of heat (microbial decomposition) and why there are sometimes spontaneous silo fires (improperly dried/prepared silage). But don't worry - there is no such thing as spontaneous human combustion.
Many of the enzymes of glycolysis and respiration are allosteric enzymes. This allows for tight control of metabolism.
Last updated: July 14, 2009 © Copyright by SG Saupe