Special Sub-Topic: Bacterial Metabolism Click Here To Play: Bacterial Metabolism

Substrate level phosphorylation can occur both in aerobic and anaerobic environments.

t. Substrate level phoshorylation, which is associated with fermentation, involves ATP generation without the need for an externally supplied electron acceptor (e.g. oxygen) since the electron acceptor (often an organic acid) is a product of the pathway itself. The ATP yield of substrate level phosphorylation is much lower than that of oxidative phosphorylation, which is associated with respiration.

The Embden-Meyerhof and Entner - Douderoff pathways are two related pathways that are also called _________?

Glycolysis. Embden-Meyerhoff is the predominant form of glycolysis found throughout the bacterial kingdom. The Entner-Douderhoff pathway is common in aerobic Gram - negative bacteria, but rare in Gram - positive and/or anaerobic bacteria. Both pathways catalyze the oxidation of one mole of glucose to two moles of pyruvic acid.

The tricarboxylic acid (AKA Kreb's or Citric Acid) Cycle converts acetate to carbon dioxide and NADH in a series of reactions that convert six carbon compounds to five carbon then to four carbon compounds. Oxalacetate, a four carbon compound, rejoins with acetate (a two carbon compound) to start the cycle again. Which of the following compounds is NOT associated with the cycle?

Propionate. Propionic acid / propionate, although a carboxylic acid, is a three carbon compound not associated with the Kreb's Cycle. The compounds alpha - ketoglutarate and oxaloacetate are important, as they are precursors to several amino acids.

Which of the following compounds are electron acceptors used in anaerobic respiration?

All of these. (Nitrate., Sulfate., Fumarate.). All of those compounds can be used as electron acceptors by certain groups of bacteria for anaerobic respiration. Reductases catalyze the reactions. Even the ubiquitous and well studied E. coli can readily respire anaerobically using nitrate as its terminal electron acceptor. While many organisms can assimilate nitrate or sulfate as nitrogen or sulfur sources, respectively, utilizing those compounds in an dissimilative (as electron acceptors in energy metabolism) manner is restricted to the prokaryotes. Fumarate can also be used an an respiratory electron acceptor in certain facultative anaerobes and anaerobes.

When a bacteria oxidizes inorganic compounds as its source of energy while utilizing carbon dioxide as its sole source of carbon, it is said to be a(n):

chemolithotroph. Although chemolithotrophs have electron transport chains (or derivatives thereof) and derive ATP through a proton motive force, the distinction between a chemolithotroph and a chemoheterotroph is the carbon source. Chemoheterotrophs use organic compounds (e.g. glucose) as their carbon source for biomass and energy production. Chemolithotrophs utilize carbon dioxide for both. Opportunistic pathogenic bacteria such as E. coli, Salmonella spp., Pseudomonas spp. as well as most currently characterized bacteria are chemoheterotrophs. A bacterium crucial to the nitrogen cycle, Nitrosomonas spp., is a chemolithotroph that oxidizes ammonium to nitrite. Chemolithotrophy is a type of autotrophy, which is a general term for an organism that "fixes" carbon dioxide.

Some bacteria are both chemolithotrophic and chemoheterotrophic.

t. Yes! This is called facultative chemolithotrophy or mixotrophy. In this case, either organic carbon or carbon dioxide can be used for biomass and energy production. This is a somewhat of an oversimplification, however. For example, the sulfur oxidizer Beggiatoa, for example, oxidizes inorganic sulfur compounds as its energy source, but lacks a complete pathway to utilize carbon dioxide as its carbon source for biomass. For Beggiatoa and similiar bacteria, chemolithotrophy and chemoorganotrophy complement each other.

Which of the following is the most common pathway for carbon dioxide fixation in chemolithotrophs?

Calvin Cycle. The Calvin cycle is the most widespread carbon dioxide fixation pathway in bacteria, and is also employed in the chloroplasts and eukaryotic plants and algae. The acetyl - CoA pathway is employed by methanogens, acetogenic bacteria and many autotrophic sulfate reducing bacteria. The reductive tricarboxylic acid cycle is employed by certain strict anaerobes such as Desulfobacter and Hydrogenobacter. This cycle is essentially a reversal of the common tricarboxylic acid cycle (AKA Kreb's or citric acid cycle). The hydroxypropionate pathway for carbon dioxide fixation is more recently described, and is employed by the the anoxygenic phototroph (does not produce oxygen as a byproduct of photosynthesis) Chloroflexus. Interestingly, Chloroflexus is the earliest branching anoxygenic phototroph and is believed to have evolved before the cyanobacteria. It is thus possible that the hydroxypropionate pathway for carbon dioxide fixation is a truly ancient one.

In which environment may you find a methanogenic archaebacterium employing the acetyl- CoA pathway?

Your large intestine.. Methanogens reside in very low redox (low oxygen availbility) environments. Your small intestine, skin or your hair have too high of an oxygen concentration to support growth of these strict anaerobes. Your colon, however, is largely anaerobic and low redox, and is a natural environment for methanogens.

Back to heterotrophy. When a bacterium is utilizing two or three carbon organic compounds as its carbon source, the tricarboxylic acid cycle cannot alone regenerate oxaloacetate when carbon is drawn off for biomass production. In order to grow on two or three carbon compounds, some bacteria supplement the tricarboxylic acid cycle with the:

glycoxylate cycle. The glycoxylate cycle is similar to the tricarboxylic acid cycle, although the glycoxylate cycle deviates at the isocitrate step. In the glycoxylate cycle, isocitrate is converted to succinate (which is a starting point for biosynthesis), and glycoxylate. At this point an acetyl - CoA from acetate or a three carbon compound reenters the cycle, and glycoxylate is converted to malate, a four carbon compound. Malate is converted to oxaloacetate and the cycle continues in all its glory.

Under which condition might an obligately aerobic bacterium utilizing glucose as its sole source of carbon begin to accumulate significant amounts of acetate in its medium?

Deficiency in the rate of its tricarboxylic acid cycle.. If there is a buildup of acetate, the tricarboxylic acid cycle is not keeping up with the conversion of pyruvate (from glycolysis) to acetate or with the beta oxidation of fatty acids to acetate. Normally, almost all acetate is converted to acetyl- CoA, which then enters the tricarboxylic acid cycle.