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Biochemistry-II - Amino Acid Metabolism Carbon

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    Study Guide Biochemistry - II – Amino Acid Metabolism : Carbon 1. 1 - Carbon Metabolism In our cells, many important reactions involve moving tiny chemical pieces called 1 - carbon units (like – CH ₃ , – CH ₂ – , or – CHO). These reactions are called 1 - carbon metabolism . To make them work, the body mainly depends on two helpers (cofactors): • Folic acid (folate) • Vitamin B ₁₂ Together, they help build DNA, RNA, amino acids, membrane lipids, and neurotransmitters. In other words, they support growth, repair, and normal cell function. Folic Acid: The Carrier of One - Carbon Units Why folic acid is important Folic acid helps the cell activate, carry, and transfer single - carbon groups . These reactions are essential for: • Amino acid metabolism • DNA and RNA synthesis • Making membrane lipids • Producing neurotransmitters Some medicines even target folic acid metabolism to stop cancer cells or bacteria from growing. Structure of folic acid Folic acid is made of three parts : 1. Pteridine ring (6 - methylpterin) 2. p - Aminobenzoic acid (PABA) 3. Glutamic acid

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    Study Guide The glutamic acid part does not take part directly in reactions. Instead, cells often attach several glutamates (3 – 8 of them) to folate. This forms a negatively charged “handle” that keeps folate inside the cell and helps it bind to enzymes. The pteridine and PABA parts are the ones that actually do the chemistry. Figure 1 Activating folic acid Before folic acid can work, it must be reduced twice by NADPH inside the cell. • The pyrazine ring of the pteridine portion gets reduced. • After two reductions, folic acid becomes: Tetrahydrofolate (THF) THF is the active form that accepts and carries 1 - carbon units.

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    Study Guide How THF carries carbon units Tetrahydrofolate usually accepts a carbon group from serine (and sometimes from glycine). This produces the key compound: N⁵,N¹⁰ - methylene - tetrahydrofolate Think of this as the central hub of 1 - carbon metabolism. Figure 2

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    Study Guide Different forms, different jobs N⁵,N¹⁰ - methylene - THF can change into other forms depending on what the cell needs: • Methylene form → used to make thymidine (dTMP) for DNA. • Methenyl / formyl form → used in purine synthesis for DNA and RNA. • Methyl form → used to make methionine . So, folate doesn’t just carry carbon — it adjusts the form of carbon for different pathways. Folate Antagonists: Sulfa Drugs Some drugs stop bacteria by blocking folate metabolism. Example: Sulfanilamide Sulfanilamide looks very similar to p - aminobenzoic acid (PABA) . Because of this similarity: • Bacteria mistakenly use sulfanilamide. • They can’t make folic acid properly. • Their growth is stopped. These are called sulfa drugs and were among the first antimetabolites used to treat infections. Humans are safe because we do not synthesize folic acid — we get it from food — so the drug mainly harms bacteria, not us.

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    Study Guide Figure 3 Vitamin B ₁₂ : A Critical Partner Vitamin B ₁₂ takes part in only two reactions , but they are essential for life . People who cannot absorb B ₁₂ develop pernicious anemia , which is treated today with B ₁₂ injections. Structure of vitamin B ₁₂ Vitamin B ₁₂ contains: • A cobalt (Co) ion • Bound inside a ring structure • The cobalt makes six bonds: o Four to the ring nitrogens o One to a nitrogen of dimethylbenzamidine o One to either cyanide (vitamin form) or adenosine (active coenzyme) This cobalt is what allows B ₁₂ to carry and transfer methyl groups. Role in fatty acid metabolism Vitamin B ₁₂ is needed for the enzyme:

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    Study Guide Methylmalonyl - CoA mutase This enzyme is important when breaking down: • Odd - chain fatty acids • Branched - chain amino acids Odd - chain fats produce propionyl - CoA , which becomes methylmalonyl - CoA . With the help of B ₁₂ , methylmalonyl - CoA is converted into: Succinyl - CoA Succinyl - CoA enters the Krebs cycle to produce energy. Figure 4 Role in methionine synthesis Vitamin B ₁₂ also helps make methionine . Steps: 1. 5 - methyl - THF gives a methyl group to B ₁₂ . 2. B ₁₂ holds the methyl group on cobalt. 3. B ₁₂ transfers it to homocysteine . 4. Homocysteine becomes methionine . Methionine is important for proteins and for donating methyl groups.

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    Study Guide Methyltransferases and SAM Most methyl transfers use methionine through a special molecule: S - adenosylmethionine (SAM) SAM is made from: • Methionine • ATP All three high - energy phosphate bonds of ATP are used. Why SAM works so well Methyl transfer from SAM is very favorable because: • The positive charge on sulfur is relieved. • The bond to adenosine breaks easily. • The leftover product ( S - adenosyl - homocysteine ) is quickly removed. This is important because S - adenosyl - homocysteine can block methyltransferases if it builds up.

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