Catabolism of Lipids and Proteins

Previous sections have discussed the catabolism of glucose, which provides energy to living cells, as well as how polysaccharides like glycogen, starch, and cellulose are degraded to glucose monomers. But microbes consume more than just carbohydrates for food. In fact, the microbial world is known for its ability to degrade a wide range of molecules, both naturally occurring and those made by human processes, for use as carbon sources. In this section, we will see that the pathways for both lipid and protein catabolism connect to those used for carbohydrate catabolism, eventually leading into glycolysis, the transition reaction, and the Krebs cycle pathways. Metabolic pathways should be considered to be porous—that is, substances enter from other pathways, and intermediates leave for other pathways. These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways.

Lipid Catabolism

Triglycerides are a form of long-term energy storage in animals. They are made of glycerol and three fatty acids (see Lipids). Phospholipids compose the cell and organelle membranes of all organisms, although the archaeal membrane phospholipids are ether-linked rather than ester-linked. (Source note: the source says phospholipids compose the membranes “of all organisms except the archaea.” The book’s own section on the unique characteristics of prokaryotic cells states that archaeal membrane phospholipids are formed with ether linkages rather than the ester linkages of bacterial and eukaryotic membranes, so this page follows that section.) Phospholipid structure is similar to triglycerides except that one of the fatty acids is replaced by a phosphorylated head group (see Lipids). Triglycerides and phospholipids are broken down first by releasing fatty acid chains (and/or the phosphorylated head group, in the case of phospholipids) from the three-carbon glycerol backbone. The reactions breaking down triglycerides are catalyzed by lipases and those involving phospholipids are catalyzed by phospholipases. These enzymes contribute to the virulence of certain microbes, such as the bacterium Staphylococcus aureus and the fungus Cryptococcus neoformans. These microbes use phospholipases to destroy lipids and phospholipids in host cells and then use the catabolic products for energy (see Virulence Factors of Bacterial and Viral Pathogens in Chapter 15).

The resulting products of lipid catabolism, glycerol and fatty acids, can be further degraded. Glycerol can be phosphorylated to glycerol-3-phosphate and easily converted to glyceraldehyde 3-phosphate, which continues through glycolysis. The released fatty acids are catabolized in a process called β-oxidation, which sequentially removes two-carbon acetyl groups from the ends of fatty acid chains, reducing NAD⁺ and FAD to produce NADH and FADH₂, respectively, whose electrons can be used to make ATP by oxidative phosphorylation. The acetyl groups produced during β-oxidation are carried by coenzyme A to the Krebs cycle, and their movement through this cycle results in their degradation to CO₂, producing ATP by substrate-level phosphorylation and additional NADH and FADH₂ molecules (see Appendix C for a detailed illustration of β-oxidation).

Other types of lipids can also be degraded by certain microbes. For example, the ability of certain pathogens, like Mycobacterium tuberculosis, to degrade cholesterol contributes to their virulence. The side chains of cholesterol can be easily removed enzymatically, but degradation of the remaining fused rings is more problematic. The four fused rings are sequentially broken in a multistep process facilitated by specific enzymes, and the resulting products, including pyruvate, can be further catabolized in the Krebs cycle.

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Protein Catabolism

Proteins are degraded through the concerted action of a variety of microbial protease enzymes. Extracellular proteases cut proteins internally at specific amino acid sequences, breaking them down into smaller peptides that can then be taken up by cells. Some clinically important pathogens can be identified by their ability to produce a specific type of extracellular protease. For example, the production of the extracellular protease gelatinase by members of the genera Proteus and Serratia can be used to distinguish them from other gram-negative enteric bacteria. Following inoculation and growth of microbes in gelatin broth, degradation of the gelatin protein due to gelatinase production prevents solidification of gelatin when refrigerated. Other pathogens can be distinguished by their ability to degrade casein, the main protein found in milk. When grown on skim milk agar, production of the extracellular protease caseinase causes degradation of casein, which appears as a zone of clearing around the microbial growth. Caseinase production by the opportunist pathogen Pseudomonas aeruginosa can be used to distinguish it from other related gram-negative bacteria.

After extracellular protease degradation and uptake of peptides in the cell, the peptides can then be broken down further into individual amino acids by additional intracellular proteases, and each amino acid can be enzymatically deaminated to remove the amino group. The remaining molecules can then enter the transition reaction or the Krebs cycle.

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Summary

• Collectively, microbes have the ability to degrade a wide variety of carbon sources besides carbohydrates, including lipids and proteins. The catabolic pathways for all of these molecules eventually connect into glycolysis and the Krebs cycle.
• Several types of lipids can be microbially degraded. Triglycerides are degraded by extracellular lipases, releasing fatty acids from the glycerol backbone. Phospholipids are degraded by phospholipases, releasing fatty acids and the phosphorylated head group from the glycerol backbone. Lipases and phospholipases act as virulence factors for certain pathogenic microbes.
• Fatty acids can be further degraded inside the cell through β-oxidation, which sequentially removes two-carbon acetyl groups from the ends of fatty acid chains.
• Protein degradation involves extracellular proteases that degrade large proteins into smaller peptides. Detection of the extracellular proteases gelatinase and caseinase can be used to differentiate clinically relevant bacteria.

Key terms

• lipases — extracellular enzyme that degrades triglycerides.
• phospholipases — enzyme that degrades phospholipids.
• β-oxidation — process of fatty acid degradation that sequentially removes two-carbon acetyl groups, producing NADH and FADH₂, on entry into the Krebs cycle.
• protease — enzyme that cuts proteins into smaller peptides.

Practice

Describe how lipids are catabolized

Describe how lipid catabolism can be used to identify microbes

Describe how proteins are catabolized

Describe how protein catabolism can be used to identify bacteria

This section is adapted from Microbiology, Section 8.5: Catabolism of Lipids and Proteins by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: this section has no figures and no CALS table; the two self-closing same-passage cross-references to Section 7.3’s triglyceride and phospholipid figures (<link document="m58815">) are rendered as two Markdown links to the Lipids section page, with link text supplied from that section’s title (a describing phrase supplied by the author, since the source left both links unnamed); the cross-reference to Appendix C (m58948, not authored) is left as plain text naming the appendix; the cross-reference to chapter 15’s Virulence Factors of Bacterial and Viral Pathogens (m58868, not authored) is left as plain text naming the chapter and section title; the Clinical Focus box is rendered as a callout, its “Jump to the next / Go back to the previous Clinical Focus box” links replaced by a plain sentence naming that the case continues in Biogeochemical Cycles and began in Energy, Matter, and Enzymes; both of the section’s body Check Your Understanding bullets remain self-checks (body graded 0, self-check 2): the first (how lipases and phospholipases contribute to virulence) needs two separate module sentences — the general “contribute to the virulence of certain microbes” sentence and the more specific phospholipase-mechanism sentence — assembled together, since the module states the destroy-host-lipids-for-energy mechanism only for phospholipases, not lipases by name; the second (how protein catabolism identifies microbes) remains a self-check because its honest answer assembles the general principle with both the gelatinase and caseinase examples; the three source Multiple Choice and three source Fill in the Blank items keep the source’s own keys, option order, and wording, except that the “ATP by oxidative phosphorylation” fill-in-the-blank also accepts the shorter answer “ATP” (adjudicated: a learner who fills the blank with only “ATP” is not wrong, even though the source’s fuller phrase stays the primary key); of the section’s two unkeyed Short Answer questions, both remain self-checks with model answers and rubrics assembled only from this module’s own sentences, because each honest answer requires assembling facts from across both the Lipid Catabolism and Protein Catabolism subsections; the unkeyed Critical Thinking question (whether β-oxidation requires cellular respiration) remains a self-check whose model answer states only what the module gives about β-oxidation feeding oxidative phosphorylation and the Krebs cycle, and explicitly stops there because the module never states whether the pathway requires a functioning respiratory chain; five filler items (two multiple-choice items on which pathogen is linked to which lipid-degrading virulence trait, one cloze textin naming pyruvate from the module’s own cholesterol-ring-degradation sentence — chosen over an earlier draft that clozed “phospholipases,” which turned out to duplicate a literal word in the source-verbatim body Check Your Understanding question above it — and two multiple-choice items on the gelatinase/caseinase identification tests, the second of which draws its two wrong-context distractors from the module’s own Clinical Focus test-result sentences, gray nonhemolytic colonies and a positive oxidase test, rather than invented results) round out the “identify microbes” and “identify bacteria” Practice groups, which the section’s own six keyed exercises do not reach on their own; no source exercise was omitted; key terms are compiled from the module’s four defined terms and the book’s Glossary appendix, except for protease, whose Glossary sense (“removes individual amino acids from the ends of peptide chains,” an exopeptidase description) does not match this module’s own sentence, which describes extracellular proteases as cutting proteins internally into smaller peptides (an endopeptidase description); the Key terms bullet for protease is therefore sentence-derived from the module’s own text instead of the Glossary appendix. One claim is corrected against the book itself: the source’s statement that phospholipids compose the membranes of all organisms “except the archaea” contradicts the book’s own account of ether-linked archaeal membrane phospholipids, so the sentence is corrected with a visible Source note (erratum 512).