Some bacteria require p -aminobenzoate in the culture medium...

Some bacteria require $p$ -aminobenzoate in the culture medium for normal growth, and their growth is severely inhibited by the addition of sulfanilamide, one of the earliest sulfa drugs. Moreover, in the presence of this drug, 5 -aminoimidazole-4-carboxamide ribonucleotide (AICAR; see Fig. $22-35$ ) accumulates in the culture medium. These effects are reversed by addition of excess $p$ aminobenzoate. (a) What is the role of $p \text { -aminobenzoate in these bacteria? (Hint: See Fig. } 18-16 .)$ (b) Why does AICAR accumulate in the presence of sulfanilamide? (c) Why are the inhibition and accumulation reversed by addition of excess $p$ aminobenzoate?

Some bacteria require $p$ -aminobenzoate in the culture medium for normal growth, and their growth is severely inhibited by the addition of sulfanilamide, one of the earliest sulfa drugs. Moreover, in the presence of this drug, 5 -aminoimidazole-4-carboxamide ribonucleotide (AICAR; see Fig. $22-35$ ) accumulates in the culture medium. These effects are reversed by addition of excess $p$ aminobenzoate.
(a) What is the role of $p \text { -aminobenzoate in these bacteria? (Hint: See Fig. } 18-16 .)$
(b) Why does AICAR accumulate in the presence of sulfanilamide?
(c) Why are the inhibition and accumulation reversed by addition of excess $p$ aminobenzoate?

Key Concepts

Competitive Inhibition

Competitive inhibition is a process in which a compound resembling a substrate binds to an enzyme’s active site, thereby blocking the access of the natural substrate. In this context, sulfanilamide acts as a competitive inhibitor by mimicking p-aminobenzoate, thus binding to dihydropteroate synthase and preventing the enzyme from carrying out its normal role in folate synthesis. This inhibition disrupts the production of folate and, as a consequence, various downstream metabolic reactions.

Purine Synthesis and AICAR Accumulation

Purine synthesis is a pathway responsible for generating the purine nucleotides essential for DNA and RNA synthesis. AICAR is an intermediate produced during this complex process. When folate biosynthesis is inhibited by sulfanilamide, the necessary folate derivatives that donate formyl groups in one of the later steps of purine synthesis become unavailable, leading to the accumulation of the intermediate AICAR. This accumulation indicates a bottleneck caused by inadequate folate-mediated transformations in the purine biosynthetic pathway.

Folate Biosynthesis

Folate biosynthesis is the metabolic pathway that produces folic acid from simpler precursor molecules. p-Aminobenzoate (pABA) serves as a vital substrate in this pathway, being incorporated into dihydropteroate and subsequently folate. Since folate derivatives are essential for transferring one-carbon units in the synthesis of nucleotides and certain amino acids, disruption of this pathway can severely hamper cellular proliferation and metabolism.

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