ch18 nitrogen metabolism

Nitrogen metabolism primarily focuses on which element’s fate in the body?

Nitrogen

Nitrogen is essential for all of the following EXCEPT:

Anything other than amino acids, nucleotides, or coenzymes

What is the major reason nitrogen metabolism must be tightly regulated?

Ammonia toxicity

Approximately what percentage of the Earth’s atmosphere is N₂?

78%

Humans cannot directly use atmospheric N₂ because:

Humans cannot take nitrogen from the atmosphere and rely on bacteria to process it

Nitrogen fixation is carried out primarily by:

Nitrogen-fixing bacteria

In plants, nitrogen-fixing bacteria are commonly found:

In the roots

“Fixing” nitrogen refers to:

Converting atmospheric nitrogen to a usable biological form

The enzyme responsible for reducing atmospheric N₂ to NH₃ is:

Nitrogenase

Nitrogenase-catalyzed nitrogen fixation:

Produces ammonia (NH₃)

Nitrogenase requires which of the following conditions?

Anaerobic conditions

The nitrogenase reaction is energetically:

Energy-demanding (requires 16 ATP)

Which enzyme converts glutamate to glutamine by adding an extra amino group?

Glutamine synthetase

Glutamine can be described as an “ammonia sponge” because it:

Stores an extra amino group and safely carries ammonia

Nitrogen enters most biological molecules primarily in the form of:

Amino groups (–NH₂)

The enzyme that converts glutamine + α-ketoglutarate into 2 glutamate is:

Glutamate synthase

In the glutamate synthase reaction, which TCA intermediate is used as a carbon skeleton?

α-ketoglutarate

Together, glutamine synthetase and glutamate synthase accomplish:

Assimilation of NH₄⁺ into organic molecules

Transaminases (aminotransferases) catalyze reactions that:

Transfer amino groups between molecules

Transamination reactions are important because they:

Link amino acid and keto acid pools through reversible reactions

A typical transamination reaction involves:

Reversible transfer of amino groups between amino acids and keto acids

Transaminations are highly regulated because:

Ammonia is toxic

The main “amino group donor” in many transamination routes is:

Glutamate

Which statement best describes glutamine’s role in nitrogen metabolism?

A safe, non-toxic carrier and storage form of nitrogen

The major reason the body uses organic carriers instead of free NH₃ is to:

Prevent ammonia toxicity

Which pair represents the interconversion central to nitrogen assimilation?

Glutamate ↔ α-ketoglutarate

Which molecule is both a TCA intermediate and central in nitrogen assimilation?

α-ketoglutarate

The main metabolic danger of elevated glutamine in the brain is:

Cerebral edema

Which statement about nitrogen acquisition and disposal is TRUE?

Nitrogen acquisition + nitrogen disposal (urea cycle)

The urea cycle is most directly linked to nitrogen:

Nitrogen disposal

Amino acids are derived mainly from intermediates of:

Glycolysis, TCA cycle, pentose phosphate pathway

The primary nitrogen sources for amino acid biosynthesis are:

Glutamate and glutamine

Essential amino acids are those that:

Must be obtained from the diet

Nonessential amino acids:

Are synthesized by the body

The glutamate family of amino acids is grouped because they:

Share a 5-carbon backbone

Serine can serve as a precursor for:

Glycine

Folate is important in amino acid metabolism because it:

Carries out one-carbon transfers

Folate has a critical role in:

One-carbon transfers and purine synthesis

Essential amino acid synthesis pathways are often:

Long and energy-intensive

Humans do not synthesize sulfur-containing amino acids because:

We cannot efficiently incorporate sulfur

The branched-chain amino acids include:

Valine, leucine, isoleucine

The aromatic amino acids include:

Phenylalanine, tyrosine, tryptophan

Histidine is synthesized from:

ATP + PRPP

Glutamate, GABA, and glycine are all:

Neurotransmitters

GABA is produced from which amino acid?

Glutamate

The enzyme that converts glutamate to GABA is:

Glutamate decarboxylase

GABA’s principal effect in the nervous system is to:

Prevent overstimulation

Drugs like benzodiazepines act by:

Enhancing GABA’s inhibitory action

Catecholamines dopamine, norepinephrine, epinephrine are derived from:

Tyrosine

Parkinson’s disease is primarily associated with deficiency of:

Dopamine

A common treatment for Parkinson’s disease involves:

Dopamine agonists

Serotonin and melatonin are synthesized from:

Tryptophan

Prozac (fluoxetine) is best described as a:

Serotonin reuptake inhibitor

Amino acids can be used as metabolic fuel primarily in:

Small intestine and liver

Amino acid catabolism usually begins with:

Transamination

After transamination, the carbon skeleton of amino acids:

Enters TCA, gluconeogenesis, or ketogenesis

A glucogenic amino acid yields:

TCA cycle intermediates → glucose

A ketogenic amino acid yields:

Acetyl-CoA or acetoacetate

Ketosis occurs when:

Sugar is low and ketones become major fuel

Phenylketonuria (PKU) is characterized by:

Accumulation of phenylalanine

A key clinical feature of untreated PKU is:

Neurological impairment

PKU is typically managed by:

A low-phenylalanine diet

The main physiological purpose of the urea cycle is to:

Detoxify ammonia → urea

The urea cycle occurs primarily in:

Liver

The committed enzyme of the urea cycle is:

Carbamoyl phosphate synthetase I (CPS I)

Carbamoyl phosphate synthetase I is located in the:

Mitochondria

The first step of the urea cycle incorporates ammonia into:

Carbamoyl phosphate

In the liver, toxic ammonia is first handled in the:

Mitochondria

The ornithine–citrulline antiporter:

Moves ornithine in and citrulline out

N-acetylglutamate (NAG) regulates the urea cycle by:

Activating CPS I

A nucleotide is composed of:

Base + sugar + phosphate

A nitrogenous base attached only to a sugar is called a:

Nucleoside

Purine bases include:

Adenine and guanine

Pyrimidine bases include:

Cytosine, uracil, thymine

Purines are structurally characterized by:

A two-ring structure

Pyrimidines are structurally characterized by:

A one-ring structure

“Pure As Gold” refers to:

Purines = A, G

“CUT the PY” refers to:

Pyrimidines = C, U, T

In RNA, the sugar is:

Ribose

In DNA, the sugar is:

Deoxyribose

The phosphate attaches to which carbon?

Carbon 5

Thymine is found mainly in:

DNA

Uracil is found mainly in:

RNA

DNA uses thymine instead of uracil because:

Allows detection of cytosine deamination

Which base pair forms two hydrogen bonds?

A–T

Which base pair forms three hydrogen bonds?

G–C

Nucleotides function as all EXCEPT:

Structural lipids

cAMP and cGMP function as:

Second messengers

In de novo purine synthesis, the ring is:

Built onto ribose (PRPP)

Amino acid precursors for purine synthesis include:

Glycine, glutamine, aspartate

The common purine precursor for AMP and GMP is:

IMP

Conversion of IMP to AMP uses:

GTP

Conversion of IMP to GMP uses:

ATP

In pyrimidine synthesis, the ring is:

Built first, then attached to ribose

Primary precursors for pyrimidine synthesis include:

Aspartate, carbamoyl phosphate, CO₂

Ribonucleotide reductase converts:

Ribonucleotides → deoxyribonucleotides

Ribonucleotide reductase is allosterically regulated by:

ATP activates, dATP inhibits

Thymidylate synthase converts:

dUMP → dTMP

The methyl donor used by thymidylate synthase is:

THF

Purines degrade to:

Uric acid