4 research outputs found
Characterization of THB1, a Chlamydomonas reinhardtii truncated hemoglobin: linkage to nitrogen metabolism and identification of lysine as the distal heme ligand
The nuclear genome of the model organism Chlamydomonas reinhardtii contains genes for a dozen hemoglobins of the truncated lineage. Of those, THB1 is known to be expressed, but the product and its function have not yet been characterized. We present mutagenesis, optical, and nuclear magnetic resonance data for the recombinant protein and show that at pH near neutral in the absence of added ligand, THB1 coordinates the heme iron with the canonical proximal histidine and a distal lysine. In the cyanomet state, THB1 is structurally similar to other known truncated hemoglobins, particularly the heme domain of Chlamydomonas eugametos LI637, a light-induced chloroplastic hemoglobin. Recombinant THB1 is capable of binding nitric oxide (NO(*)) in either the ferric or ferrous state and has efficient NO(*) dioxygenase activity. By using different C. reinhardtii strains and growth conditions, we demonstrate that the expression of THB1 is under the control of the NIT2 regulatory gene and that the hemoglobin is linked to the nitrogen assimilation pathway
Facile Heme Vinyl Posttranslational Modification in a Hemoglobin
Iron-protoporphyrin
IX, or <i>b</i> heme, is utilized as such by a large number
of proteins and enzymes. In some cases, notably the <i>c</i>-type cytochromes, this group undergoes a posttranslational covalent
attachment to the polypeptide chain, which adjusts the physicochemical
properties of the holoprotein. The hemoglobin from the cyanobacterium <i>Synechocystis</i> sp. PCC 6803 (GlbN), contrary to the archetypical
hemoglobin, modifies its <i>b</i> heme covalently. The posttranslational
modification links His117, a residue that does not coordinate the
iron, to the porphyrin 2-vinyl substituent and forms a hybrid <i>b</i>/<i>c</i> heme. The reaction is an electrophilic
addition that occurs spontaneously in the ferrous state of the protein.
This apparently facile type of heme modification has been observed
in only two cyanobacterial GlbNs. To explore the determinants of the
reaction, we examined the behavior of <i>Synechocystis</i> GlbN variants containing a histidine at position 79, which is buried
against the porphyrin 4-vinyl substituent. We found that L79H/H117A
GlbN bound the heme weakly but nevertheless formed a cross-link between
His79 NĪµ2 and the heme 4-CĪ±. In addition to this linkage,
the single variant L79H GlbN also formed the native His117ā2-CĪ±
bond yielding an unprecedented bis-alkylated protein adduct. The ability
to engineer the doubly modified protein indicates that the histidineāheme
modification in GlbN is robust and could be engineered in different
local environments. The rarity of the histidine linkage in natural
proteins, despite the ease of reaction, is proposed to stem from multiple
sources of negative selection