Overview of Nitrogen Metabolism and Amino Acid Biosynthesis

Vocabulary practice flashcards covering dietary protein utilization, nitrogen excretion products, essential and nonessential amino acids, nitrogen balance, and key enzymatic pathways.

Dietary Proteins

The primary source of nitrogen that is metabolized by the human body.

Branched-Chain Amino Acids

The amino acids leucine, isoleucine, and valine, which can be oxidized for energy by most body tissues.

Urea Cycle

The metabolic process occurring in the liver that converts toxic ammonia and amino groups from amino acids into nontoxic, water-soluble urea for excretion in urine.

Urinary Urea Excretion

The major nitrogenous urinary excretory product, amounting to $12\text{--}20\text{ g}$ of urea nitrogen ($12,000\text{--}20,000\text{ mg}$) excreted per day.

Urinary Ammonium Ion ($\text{NH}_4^+$) Excretion

An excretory product formed in the kidney to help buffer urine, resulting in $140\text{--}1500\text{ mg}$ of ammonia nitrogen excreted per day.

Creatinine Excretion Rate

The daily urinary excretion rate of creatinine produced from creatine phosphate, which is $14\text{--}26\text{ mg/kg}$ in men and $11\text{--}20\text{ mg/kg}$ in women.

Uric Acid Excretion Rate

The daily urinary excretion amount of uric acid (the degradation product of purine bases), which is $250\text{--}750\text{ mg}$.

Bilirubin Excretion

A nitrogen-containing degradation product formed from heme that is excreted mainly in the feces.

Nonessential Amino Acids

The 11 amino acids synthesized in the body: alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, and serine (produced from glucose), plus tyrosine (synthesized from phenylalanine).

Cysteine Biosynthesis

Synthesis of cysteine from glucose, which requires obtaining its sulfur atom from the essential amino acid methionine.

Essential Amino Acids

The 9 amino acids whose carbon skeletons cannot be synthesized by humans and must be supplied in the diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Arginine Essentiality

An amino acid required during periods of growth (such as in children) but not required in adult diets.

Positive Nitrogen Balance

A state where nitrogen intake exceeds nitrogen excretion, occurring during tissue growth such as childhood, pregnancy, lactation, or recovery from emaciating illness.

Negative Nitrogen Balance

A state where nitrogen loss exceeds nitrogen intake, caused by inadequate protein intake, lack of an essential amino acid, trauma, burns, illness, surgery, or cancer.

Nitrogen Balance Formula

$\text{N balance} = \text{protein N intake (g/24 hrs)} - (\text{UUN} + 4\text{ g})$, where $\text{UUN}$ is urinary urea nitrogen and $4\text{ g}$ accounts for non-UUN urinary, skin, and fecal losses.

Glutamate

An amino acid central to nitrogen metabolism that provides nitrogen for amino acid synthesis, obtaining its nitrogen from other amino acids via transamination or from $\text{NH}_4^+$ via glutamate dehydrogenase (GDH).

Glutamine Synthetase

A cytoplasmic enzyme present in all cells (and surrounding the portal vein in the liver) that synthesizes glutamine from glutamate and ammonia, using $\text{ATP}$.

Glutaminase

An enzyme in the liver, kidney, and intestine that removes the amide nitrogen from glutamine to regenerate glutamate and release free ammonia.

Glutamine Synthesis and Transport Pathway

The mechanism by which peripheral tissues convert excess ammonia into glutamine via GDH and glutamine synthetase, transporting it to the liver for urea synthesis.