Nitrogen & Nucleotide Metabolism

BCH 100 β€” Introductory Biochemistry Β· Dr. Radi

build Jul 17 Β· 15:01 Β· CC BY-NC-SA 4.0 Β· owned figures (RDKit / matplotlib / PyMOL)
Dr. Radi

By the end of this unit, you can…

  • Describe amino-acid catabolism β€” transamination and oxidative deamination β€” and where the carbon skeletons go
  • Walk the urea cycle and how the body disposes of toxic ammonia as urea
  • Explain nitrogen balance and the essential amino acids
  • Identify nucleotide structure and the purine/pyrimidine bases, and outline nucleotide biosynthesis
Dr. Radi

Today's route πŸ—ΊοΈ

  1. Amino Acid Catabolism
  2. The Urea Cycle
  3. Nitrogen Balance
  4. Nucleotides & the Bases
  5. Making & Breaking Nucleotides
Dr. Radi

1 Β· Amino Acid Catabolism

"Unlike fat and sugar, protein carries nitrogen β€” and nitrogen can't just be burned. Step one of using an amino acid for fuel is prying that nitrogen off."

Dr. Radi

Two problems in one molecule

When you burn carbs or fat, the atoms are just C, H, and O β€” they leave as COβ‚‚ and water. But an amino acid also carries nitrogen in its amino group, and nitrogen has nowhere clean to go. So catabolizing an amino acid splits into two jobs. The carbon skeleton β€” an Ξ±-keto acid β€” is a normal fuel: it feeds the TCA cycle, or becomes glucose or ketones. The amino group is the problem child: pulled off as ammonia (NH₃), which is toxic, and must be shipped to the urea cycle for disposal.

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Transamination: passing the nitrogen

The cell doesn't just rip the amino group off and release ammonia everywhere β€” that would be dangerous. Instead it hands the nitrogen off in a controlled swap called transamination. An aminotransferase enzyme takes the amino acid's –NHβ‚‚ and transfers it onto Ξ±-ketoglutarate, producing glutamate and leaving the original amino acid as its Ξ±-keto acid. Here ALT does it to alanine β€” turning it into pyruvate while making glutamate. (All these enzymes use the vitamin-B₆ cofactor PLP.)

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ALT and AST: the enzymes in a blood test

The two transaminases you'll actually meet again are ALT (alanine) and AST (aspartate) β€” shown here handing aspartate's nitrogen to Ξ±-ketoglutarate. They matter clinically because they normally live inside liver cells. When the liver is damaged β€” hepatitis, fatty liver, a paracetamol overdose β€” those cells leak, and ALT and AST spill into the blood. That's why "liver enzymes" on a blood panel are these two: a routine test that reads out cell damage through the biochemistry of nitrogen handling.

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2 Β· The Urea Cycle

"Ammonia is poison β€” especially to the brain. So your liver runs a five-step loop that locks two waste nitrogens into one safe molecule you can pee out: urea."

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What to do with the nitrogen

Once transamination has funneled waste nitrogen into ammonia, the body has to get rid of it β€” and different animals solve this differently. Fish just dump ammonia straight into the water; it's toxic but they have an ocean to dilute it. Birds and reptiles convert it to uric acid, a nearly solid paste that wastes almost no water (handy inside an egg). Mammals β€” you β€” make urea: far less toxic than ammonia, freely water-soluble, built in the liver and flushed out by the kidneys. The choice is a trade-off between toxicity, water, and energy.

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The urea cycle

Making urea takes a five-step loop in the liver, and the key is that it loads two nitrogens from two different sources. The first nitrogen enters as carbamoyl phosphate (built from ammonia + COβ‚‚), which joins ornithine to make citrulline. The second nitrogen is donated by aspartate. A couple of steps later, the enzyme snips out urea β€” carrying both waste nitrogens β€” and regenerates ornithine to run the loop again. Two toxic nitrogens in, one harmless urea out.

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When the cycle jams

This loop isn't optional β€” and you can see why when it fails. If a baby is born missing one of the urea-cycle enzymes, ammonia has nowhere to go: it builds up in the blood (hyperammonemia) and, because ammonia is a potent brain toxin, causes vomiting, lethargy, seizures, and β€” untreated β€” coma. There's no way to make the missing enzyme, so treatment is about buying time: a low-protein diet to make less nitrogen, plus drugs that escort nitrogen out by an alternate route. The whole disease is a backed-up cycle.

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3 Β· Nitrogen Balance

"You don't store protein the way you store fat. So whether you're building muscle or losing it comes down to a simple ledger: nitrogen in versus nitrogen out."

Dr. Radi

The nitrogen ledger

Because the body has no dedicated protein store, dietitians track protein status with a simple accounting trick: nitrogen balance β€” the nitrogen you eat (in protein) minus the nitrogen you excrete (as urea). It comes in three states. Positive balance (in > out) means you're net building protein β€” normal in growth, pregnancy, training, and healing. Balance (in = out) is the steady state of a healthy adult. Negative balance (in < out) means you're losing protein faster than you replace it β€” and that one is a warning sign.

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Negative balance: the body eats itself

Negative nitrogen balance happens two ways β€” too little protein coming in, or too much breakdown going out. The intake side is malnutrition, starvation, or prolonged fasting. The breakdown side is stress catabolism β€” severe illness, infection, major burns, surgery, uncontrolled diabetes, and the slow muscle loss of aging. Either way the result is the same and grim: with amino acids in short supply, the body starts dismantling its own muscle protein to get them β€” wasting. It's why serious illness costs you muscle, and why nutrition is part of the treatment.

Dr. Radi

4 Β· Nucleotides & the Bases

"The molecules that store your genome are built from just three kinds of part β€” and the 'information' comes down to five nitrogen-rich rings."

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What a nucleotide is made of

Nucleic acids look complicated, but every nucleotide β€” the repeating unit of DNA and RNA β€” is just three parts snapped together: a phosphate, a five-carbon sugar, and a nitrogenous base. The phosphate and sugar alternate to form the strand's backbone; the base sticks out to the side and carries the information. (And notice the sugar–phosphate piece is exactly what makes ATP an energy molecule too β€” same parts, different job.)

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DNA vs. RNA

DNA and RNA are built the same way but differ in four telling spots. Their sugar: DNA uses deoxyribose (missing an oxygen at the 2β€² position), RNA uses ribose (which keeps that 2β€²-OH) β€” one atom, and it's what makes DNA the more stable, long-term molecule. Their bases are almost the same set, except DNA uses thymine (T) where RNA uses uracil (U). And their shape and job: DNA is a double helix that stores the genome; RNA is usually single-stranded and carries and translates the message.

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The five bases: purines vs. pyrimidines

The "information" in a nucleic acid is spelled with just five nitrogenous bases, and they come in two shapes. Purines β€” adenine (A) and guanine (G) β€” are the big ones: two fused rings. Pyrimidines β€” cytosine (C), thymine (T), and uracil (U) β€” are the small ones: a single ring. An easy way to keep them straight: "PY-rimidines are the smaller CUT" (C, U, T), and the purines are the two that are left. In the double helix, a big purine always pairs with a small pyrimidine.

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5 Β· Making & Breaking Nucleotides

"Cells build nucleotides two different ways, convert them into DNA's building blocks with a single pivotal enzyme, and β€” when they break the leftovers down β€” occasionally give someone gout."

Dr. Radi

Building the two kinds of base

Cells make nucleotides from scratch (de novo) by two opposite strategies. For purines (A, G), the ring is built directly onto the sugar β€” atom by atom, starting from an activated ribose called PRPP β€” to make IMP, which then becomes AMP or GMP. For pyrimidines (C, U, T), it's reversed: the ring is built first (as orotate), then attached to PRPP to make UMP, and onward to CTP and dTMP. (In practice, most cells prefer the cheaper salvage route β€” recycling free bases rather than building new ones.)

Dr. Radi

From RNA parts to DNA parts

Notice that everything so far makes ribonucleotides β€” the units of RNA, with a 2β€²-OH on the sugar. So how does a cell get the deoxy version for DNA? One enzyme does it: ribonucleotide reductase, which reduces the 2β€²-OH to 2β€²-H. It's the single gateway to every DNA building block β€” which makes it a choke point. Fast-dividing cells (like a tumor, or a replicating virus) need huge amounts of dNTPs, so ribonucleotide reductase is a favorite drug target (e.g. hydroxyurea).

Dr. Radi

The leftovers: uric acid and gout

Finally, what happens to old purines? They're broken down β€” through xanthine β€” to uric acid, which you normally excrete. The catch: uric acid is barely soluble. If you make too much or excrete too little, it precipitates into needle-sharp crystals that lodge in a joint (classically the big toe), triggering the sudden, brutal inflammation of gout. The fix follows straight from the pathway: allopurinol blocks xanthine oxidase, the enzyme that makes uric acid β€” less uric acid, no crystals. Metabolism, all the way down to a sore toe.

Dr. Radi

Can you…?

  • ☐ describe amino-acid catabolism β€” transamination and oxidative deamination β€” and where the carbon skeletons go?
  • ☐ walk the urea cycle and how the body disposes of toxic ammonia as urea?
  • ☐ explain nitrogen balance and the essential amino acids?
  • ☐ identify nucleotide structure and the purine/pyrimidine bases, and outline nucleotide biosynthesis?

If any box stays empty, the practice site has a drill for it. πŸ§ͺ

Dr. Radi