Novel route to haem: pigmented chemistry

Making haem

Researchers in the UK and Portugal have used NMR spectroscopy to reveal a new way in which nature makes haem - the component that gives blood its colour and allows red blood cells to carry oxygen around the body.

Haem is at the heart of haemoglobin. It is an iron-centred porphyrin (a modified tetrapyrrole) ring bound into the blood protein and is the active centre for binding to oxygen. Such tetrapyrroles are not restricted to mammalian haemoglobin, however, they have many diverse biological functions in almost all living things. They are central to vitamin B₁₂, to coenzyme F₄₃₀, and to chlorophyll. As such, in addition to the transport of diatomic gases, they are present in enzymes, receptors and act as agents of electron transfer for redox biochemistry, with the tetrapyrrole unit acting as either a source or a sink for electrons depending on the oxidation state of its central metal ion.

Pigment of life

However, until now scientists had assumed that this vital pigment was made in a particular way. Now, Martin Warren of the University of Kent and colleagues have used a new high-power NMR machine in their department to reveal that haem can be synthesised from the related molecule sirohaem through a rather unusual and unanticipated biochemical process. Warren has described the discovery as being akin to observing how the first pocket electronic calculators were essentially reformulated into the modern smart phone.

Within funding from the Biotechnology and Biological Sciences Research Council (BBSRC) Warren and colleagues at the University of Oxford, the University of Newcastle-upon-Tyne and Portugal's Instituto de Tecnologia Química e Biológica (ITQB) have taken a close look at how Archaea, single-celled organisms forming a distinct group from prokaryotes and bacteria, biosynthesise haem from sirohaem. The research team also included Shilpa Bali of the University of Oxford and the University of Kent, Stuart Ferguson (Oxford), Andrew Lawrence, David Palmer, Mark Howard (Kent), Susana Lobo (Kent and Universidade Nova de Lisbon), Lígia Saraiva (Lisbon) and Bernard Golding of the University of Newcastle.

The team used the state-of-the-art anaerobic facilities on Kent's Canterbury campus to show that Archaea, hijack sirohaem and conscript it into the chemical process for producing haem. The team explain that this is a rare example in molecular and cellular biochemistry of one protein prosthetic group, sirohaem, effectively being cannibalised to synthesise another, haem. Sirohaem is more commonly thought of as the prosthetic group in denitrification and sulfate-reducing enzymes sulfite and nitrite reductase.

Haem transformation

The study has shown that the initial step of the transformation to haem involves the decarboxylation of the sirohaem group to generate didecarboxysirohaem. For the microbe to then produce d1 haem from this intermediate it must next replace two of the compound's propionate side chains with oxygen functionalities and then introduce a double bond into an additional peripheral side chain. The team further explains that for haem synthesis the didecarboxysirohaem is converted into iron-coproporphyrin by removal of two acetic acid side chains through oxidation. The iron-coproporphyrin intermediate in this latter case is next transformed into haem by oxidative decarboxylation of its two propionate side chains.

"This is a very important piece of basic science that offers an explanation as to how biochemical pathways evolve and become more complex," explains Warren. The team puts the sirohaem conversions into evolutionary context as well as alluding to an additional role for sirohaem. It is possible that the reverse process in which haem is hijacked by enzymes to produce sirohaem for assimilation into nitrite reductase may also have occurred.

"Moreover, we have learnt some new concepts about how chemistry can be used to change the shape and the character of larger molecules, which can then be applied for the development of new compounds; for instance, in the pharmaceutical industry or the production of biofuels. In this respect our research contributes to the field of synthetic biology."

Warren told SpectroscopyNOW that, "The next step in our research is to try and understand the mechanism of the individual steps in the process. This is important as several of these steps involve some very unusual chemistry and we would like to know how these steps are facilitated in order to try and apply them to other processes." Such processes might include production of biofuels or other high-value chemicals. "In essence, we want to be able to apply these reactions to new processes in a synthetic biology program," he adds.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.