A Mechanistic Investigation of the C-Terminal Redox Motif of Thioredoxin Reductase from Plasmodium falciparum

High-molecular mass thioredoxin reductases (TRs) are pyridine nucleotide disulfide oxidoreductases that catalyze the reduction of the disulfide bond of thioredoxin (Trx). Trx is responsible for reducing multiple protein disulfide targets in the cell. TRs utilize reduced β-nicotinamide adenine dinucleotide phosphate to reduce a bound flavin prosthetic group, which in turn reduces an N-terminal redox center that has the conserved sequence CICVNVGCCT, where CIC is denoted as the interchange thiol while the thiol involved in charge-transfer complexation is denoted as CCT. The reduced N-terminal redox center reduces a C-terminal redox center on the opposite subunit of the head-to-tail homodimer, the C-terminal redox center that catalyzes the reduction of the Trx-disulfide. Variations in the amino acid sequence of the C-terminal redox center differentiate high-molecular mass TRs into different types. Type Ia TRs have tetrapeptide C-terminal redox centers of with a GCUG sequence, where U is the rare amino acid selenocysteine (Sec), while the tetrapeptide sequence in type Ib TRs has its Sec residue replaced with a conventional cysteine (Cys) residue and can use small polar amino acids such as serine and threonine in place of the flanking glycine residues. The TR from Plasmodium falciparum (PfTR) is similar in structure and mechanism to type Ia and type Ib TRs except that the C-terminal redox center is different in its amino acid sequence. The C-terminal redox center of PfTR has the sequence G534CGGGKCG541, and we classify it as a type II high-molecular mass TR. The oxidized type II redox motif will form a 20-membered disulfide ring, whereas the absence of spacer amino acids in the type I motif results in the formation of a rare eight-membered ring. We used site-directed mutagenesis and protein semisynthesis to investigate features of the distinctive type II C-terminal redox motif that help it perform catalysis. Deletion of Gly541 reduces thioredoxin reductase activity by ∼50-fold, most likely because of disruption of an important hydrogen bond between the amide NH group of Gly541 and the carbonyl of Gly534 that helps to stabilize the β-turn-β motif. Alterations of the 20-membered disulfide ring either by amino acid deletion or by substitution resulted in impaired catalytic activity. Subtle changes in the ring structure and size caused by using semisynthesis to substitute homocysteine for cysteine also caused significant reductions in catalytic activity, demonstrating the importance of the disulfide ring's geometry in making the C-terminal redox center reactive for thiol-disulfide exchange. The data suggested to us that the transfer of electrons from the N-terminal redox center to the C-terminal redox center may be rate-limiting. We propose that the transfer of electrons from the N-terminal redox center in PfTR to the type II C-terminal disulfide is accelerated by the use of an "electrophilic activation" mechanism. In this mechanism, the type II C-terminal disulfide is polarized, making the sulfur atom of Cys540 electron deficient, highly electrophilic, and activated for thiol-disulfide exchange with the N-terminal redox center. This hypothesis was investigated by constructing chimeric PfTR mutant enzymes containing C-terminal type I sequences GCCG and GCUG, respectively. The PfTR-GCCG chimera had 500-fold less thioredoxin reductase activity than the native enzyme but still reduced selenocystine and lipoic acid efficiently. The PfTR-GCUG chimera had higher catalytic activity than the native enzyme with Trx, selenocystine, and lipoic acid as substrates. The results suggested to us that (i) Sec in the mutant enzyme accelerated the rate of thiol-disulfide exchange between the N- and C-terminal redox centers, (ii) the type II redox center evolved for efficient catalysis utilizing Cys instead of Sec, and (iii) the type II redox center of PfTR is partly responsible for substrate recognition of the cognate PfTrx substrate relative to noncognate thioredoxins.