Chapter 17

Describe three processes by which nitrogen gas \(\left(\mathrm{N}_{2}\right)\) is incorporated into the biosphere.

What three mechanisms have evolved in nitrogen-fixing microorganisms to limit the inhibitory effects of \(\mathrm{O}_{2}\) on the nitrogenase reaction?

Describe how the nitrogen cycle works to redistribute nitrogen in Earth's biosphere.

How does the combined activity of glutamine synthetase and glutamate synthase assimilate \(\mathrm{NH}_{4}^{+}\) into amino acids? Why doesn't the glutamate dehydrogenase reaction contribute much to ammonia assimilation under normal physiologic conditions?

Describe the biochemical function of the \(\mathrm{E} 1, \mathrm{E} 2,\) and E3 enzymes in the ubiquitinating system. Why are there -500 E3 genes in the human genome but only two E1 genes?

What is the difference between a glucogenic and a ketogenic amino acid? Which amino acids fit the description of both a glucogenic and a ketogenic amino acid?

What is meant by the term "inborn errors of metabolism"? Give three examples of inborn errors that are directly related to the metabolism of phenylalanine and tyrosine.

What is the functional definition of essential and nonessential amino acids in the human diet? How might this functional definition be explained in evolutionary terms?

Glyphosate (Roundup) is a very effective herbicide that is toxic to almost all types of plants. a. Why is glyphosate considered an animal-safe herbicide? b. Why does it take up to 7 days before there is any noticeable herbicidal effect of glyphosate? c. Why don't Roundup Ready soybean plants die when they are sprayed with glyphosate?

Phenylketonuria is caused by a recessive genetic mutation, whereas acute intermittent porphyria is caused by a dominant genetic mutation. a. What is the biochemical difference in terms of protein function that distinguishes most recessive mutations from dominant mutations? b. What is the probability that a father with phenylketonuria and a homozygous normal mother will have a child that is a phenylketonuria carrier? c. What is the probability that a heterozygous mother with acute intermittent porphyria will have a child with the disease if the father is disease free?