Sulfite and nitrite reductases (SiRs/NiRs) use siroheme, an iron-containing rnisobacteriochlorin, alongside a [4Fe-4S] cluster to perform the six-electron reduction of sulfite to sulfide or nitrite to ammonia. X-ray crystallographic structures of the catalytic siroheme-containing subunit of...

The phytochromes and related prokaryotic photoreceptors utilise a linear tetrapyrrole chromophore to monitor the surrounding light environment. They are found in most photosynthetic organisms, and also some nonphotosynthetic bacteria from which they most likely evolved. A key feature of the...

The regulatory network that controls formation of the various components of the photosynthetic machinery becomes evident when late steps of chlorophyll biosynthesis are investigated by deregulation. A major regulatory point is revealed by a dark-to-light shift revealing the interplay between the...

Regulation of the heme biosynthetic pathway in mammals occurs via two distinct rnmechanisms. These mechanisms reflect the fact that while most cells need to closely regulate relatively low levels of intracellular heme, differentiating erythroid cells must produce massive amounts of heme during a...

Cobalamins are complex organometallic cofactors essential for catalysis in three enzyme families, the isomerases, methyltransferases and reductive dehalogenases. This account focuses on the isomerases, which catalyze difficult and unusual 1,2-rearrangement reactions. These enzymes utilize AdoCbl...

Expositions on the regulation of biochemical pathways usually succeed in disappointing at least half of their potential audience. From a holistic standpoint, one could view gene regulation as the embodiment of the physiological significance of the encoded gene products. If one understood when,...

The destructive photosensitising ability of porphyrins in patients suffering from rnporphyria has been well documented. Patients with those porphyrias that accumulate high levels of porphyrins in the skin suffer from a range of light-activated skin manifestations, the type and severity of which...

Bilins are linear tetrapyrroles that function as the chromophores of the light-harvesting phycobiliproteins and light-signalling phytochromes in photosynthetic organisms. The biosynthesis of bilins proceeds through a ferredoxin-dependent heme oxygenase that oxidises heme to biliverdin IXa, the...

Heme oxidation catalyzed by heme oxygenase has evolved to carry out a number of rnimportant and diverse physiological processes including iron reutilization and cellular signaling in mammals, synthesis of light-harvesting pigments in cyanobacteria, light perception in plants, and the acquisition...

Heme (iron-protoporphyrin IX), as well as being an essential cofactor for cytochromes, oxidases and oxygen-binding proteins, is an important biological regulator acting via noncovalent, reversible interactions with a variety of proteins. These include DNA-binding transcription factors (e.g.,...

5-Aminolevulinic acid (ALA) is the general precursor of all known tetrapyrroles. Currently, two different biosynthetic routes for ALA formation are known. Humans, animals, fungi and the a-group of the proteobacteria employ the one-step-condensation of succinyl-coenzyme A and glycine catalyzed by...

Porphyrins are the extroverts of chemistry. Bright purple and fluorescent, they are rnused biologically in the processes of energy capture and utilization. Porphyrins are the key to life. It has been suggested that abiotic formation of porphyrins, in particular uroporphyrinogen would have...

Heme d1 is found only in a bacterial periplasmic enzyme, cytochrome cd1, that catalyses reduction of nitrite to nitric oxide. The unique features of the d1 heme include saturation of two of the pyrrole rings and presence of two carbonyl groups. Reasons for the selection of the d1 ring for an...

Cobamides are unique cyclic tetrapyrroles because their structures include an upper (Cob) and a lower (Coa) axial ligand. The Coa and Cob ligands are important for the interaction of the cobamide with enzymes, and for the chemistry of the reaction catalyzed by the enzyme. B12...

The (bacterio)chlorophyll biosynthetic pathway is of profound importance to the rnbiosphere. During the past 20 years, there have been major advances in the understanding of the genes involved in the pathway and, more recently, in the enzymes that they encode. Chlorophyll biosynthesis can be...

The porphyrias are inherited metabolic disorders resulting from partial deficiency of enzymes of the haem biosynthetic pathway. Each particular enzyme deficiency gives rise to increased levels of metabolites prior to the pathway blockage, which result in characteristic clinical features, and...

Heme proteins are located in different compartments and organelles of pro- and \r\neukaryotic cells. For their biosynthesis and assembly heme needs to be delivered to the polypeptides at the correct location. As heme is a hydrophobic molecule, it readily partitions into membranes and is bound to...

Chlorophyll metabolism is probably the most visible manifestation of life. In spite of its obvious ecological importance, chlorophyll catabolism has remained an enigma until about twelve years ago. Contrary to all expectations, chlorophyll breakdown in vascular plants rapidly leads to colorless...

The three enzymes 5-aminolaevulinic acid dehydratase (ALAD, E.C.4.2.1.24; sometimes referred to as porphobilinogen synthase), porphobilinogen deaminase (EC 4.3.1.8; also known as hydroxymethylbilane synthase) and uroporphyrinogen III synthase (U3S; E.C.4.2.1.75) together convert 5-aminolaevulinic...

Regulation of the tetrapyrrole pathway in plants is particularly crucial, since it is required for efficient synthesis of the photosynthetic apparatus, protection from the harmful phototoxicity of the pathway intermediates, and because of the proposed role played by some of the intermediates in...

Haem is an essential cofactor for virtually all organisms. It is made from the common tetrapyrrole progenitor, uroporphyrinogen III, by four sequential enzymes: uroporphyrinogen III decarboxylase, coproporphyrinogen III oxidase, protoporphyrinogen IX oxidase, and ferrochelatase. Each of the...

Methane production by methanogenic archaea takes place in many anaerobic microbial habitats such as swamps, rice paddies, fresh water sediments and the intestinal tract of animals and insects. Almost all species of methanogenic archaea are able to oxidize H2 and to use CO2 as the electron...

Vitamin B12 is a cobalt-containing modified tetrapyrrole, whose structural complexity rnand beguiling chemistry has fascinated scientists for over 80 years. As with all modified tetrapyrroles, its structure is derived from uroporphyrinogen III. This transformation requires a large number of...

Heme cofactors are found in a wide range of proteins, where they play various roles in e.g., steroid and bioactive lipid synthesis, energy transduction, gene regulation, cellular signaling, oxygen transport and antibiotic biosynthesis. The diversity of physiological functions performed by...

The biosynthesis of siroheme from uropoprhyirnogen III in bacteria, yeasts and plants is described. The pathway requires the bis-methylation of uroporphyrinogen III to generate precorrin-2, which is then oxidised to sirohydrochlorin prior to its ferrochelation. A number of structures of the...