Nav: Home

Hydrogen transfer: One thing after the other

February 14, 2018

Hydride transfer is an important reaction for chemistry (e.g., fuel cells), as well as biology (e.g., respiratory chain and photosynthesis). Often, one partial reaction involves the transfer of a hydride ion (H(-)). But does this hydride transfer involve one step or several individual steps? In the journal Angewandte Chemie, scientists have now provided the first proof of stepwise hydride transfer in a biological system.

An important step in the biosynthesis of chlorophyll is the light-dependent hydrogenation of protochlorophyllide to chlorophyllide. This involves the reduction of a double bond between carbon atoms 17 and 18 in this complex ring system to a single bond as both carbon atoms bind to an additional hydrogen atom. This step is catalyzed by the enzyme protochlorophyllide oxireductase and requires irradiation with light. Technically speaking, however, this reaction does not add one hydrogen atom to each carbon. Instead, there is first addition of a hydride ion (H(-)) to C17 and then addition of a proton (H(+)) to C18. The first partial reaction, the hydride transfer, requires the cofactor nicotinamide adenine dinucleotide phosphate (NADPH). NADPH serves as a source for two electrons and a proton (H(+)), the equivalent of a hydride anion, H(-).

Hydride transfer reactions play a key role in many biological systems. However, their mechanism is still disputed. Do the three elementary steps--transfer of an electron, a proton, and another electron from NADPH to the substrate--occur simultaneously, or stepwise?

Because of the short lifetime of the intermediates, direct proof of a stepwise mechanism has not previously been possible. Light-dependent reactions--such as the hydrogenation that occurs in the biosynthesis of chlorophyll--that can be triggered by a short laser pulse have solved this problem. By using time-resolved absorption and emission spectroscopy, researchers working with Roger J. Kutta and Nigel S. Scrutton at the University of Manchester (UK) have been able to characterize the mechanism of this hydride transfer.

In addition to excited states of protochlorophyllide, the researchers were able to resolve three discrete intermediates that are consistent with a partially stepwise mechanism: an initial electron transfer from NADPH to protochlorophyllide that has been excited (to the singlet state) by light is followed by the coupled transfer of a proton and an electron. As expected, the final step is transfer of the second proton.

Interestingly, the researchers found different intermediates for the wild type of the enzyme and a mutated version (C226S): While the initial hydride binds to C17 in the wild type, it is transferred to C18 in the mutant version. However, the end result is the same chlorphyllide stereoisomer.

The insights gained from these experiments provide a deeper understanding of how light energy can be used for chemical reactions involving hydrogen transfer, particularly with regard to the design of light-activated catalysts.
About the Author

Nigel Scrutton is a full professor in the School of Chemistry at the University of Manchester and is the Director of the Manchester Institute of Biotechnology (MIB). His main research interests are in the mechanisms and structures of enzymes, with particular reference to redox and cofactor dependent enzyme catalysis. He also has major interests in biological photochemistry, biocatalysis and metabolic engineering. He is a Fellow of the Royal Society of Chemistry and the Royal Society of Biology and has received a number of research awards and prizes over a research career spanning 30 years.


Related Fuel Cells Articles:

Paving the way for hydrogen fuel cells
The hype around hydrogen fuel cells has died down, but scientists have continued to pursue new technologies that could enable such devices to gain a firmer foothold.
Ruthenium rules for new fuel cells
Rice University scientists have fabricated a durable catalyst for high-performance fuel cells by attaching single ruthenium atoms to graphene.
Multifunctional catalyst for poison-resistant hydrogen fuel cells
A Kyushu University-led collaboration developed a catalyst that can oxidize both hydrogen and carbon monoxide in fuel cells.
Electrocatalyst nanostructures key to improved fuel cells, electrolyzers
Purdue University scientists' simulations have unraveled the mystery of a new electrocatalyst that may solve a significant problem associated with fuel cells and electrolyzers.
Nanoalloys 10 times as effective as pure platinum in fuel cells
A new type of nanocatalyst can result in the long-awaited commercial breakthrough for fuel cell cars.
Resilient red blood cells need fuel to adapt their shape to the environment
An international research team led by Osaka University built a novel 'Catch-Load-Launch' microfluidic device to monitor the resilience of red blood cells after being held in a narrow channel for various periods of time.
Cancer immunotherapy: Revived T cells still need fuel
Drugs targeting the PD-1 pathway are often described as 'releasing the brakes' on killer T cells.
Fuel cells with PFIA-membranes
HZB scientists have teamed up with partners of 3M Company in order to explore the water management in an alternative proton exchange membrane type, called PFIA.
New class of fuel cells offer increased flexibility, lower cost
A new class of fuel cells based on a newly discovered polymer-based material could bridge the gap between the operating temperature ranges of two existing types of polymer fuel cells, a breakthrough with the potential to accelerate the commercialization of low-cost fuel cells for automotive and stationary applications.
Pancreatic cancer cells find unique fuel sources to keep from starving
Pancreatic cancer cells avert starvation in dense tumors by ordering nearby support cells to supply them with an alternative source of nutrition.

Related Fuel Cells Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Climate Crisis
There's no greater threat to humanity than climate change. What can we do to stop the worst consequences? This hour, TED speakers explore how we can save our planet and whether we can do it in time. Guests include climate activist Greta Thunberg, chemical engineer Jennifer Wilcox, research scientist Sean Davis, food innovator Bruce Friedrich, and psychologist Per Espen Stoknes.
Now Playing: Science for the People

#527 Honey I CRISPR'd the Kids
This week we're coming to you from Awesome Con in Washington, D.C. There, host Bethany Brookshire led a panel of three amazing guests to talk about the promise and perils of CRISPR, and what happens now that CRISPR babies have (maybe?) been born. Featuring science writer Tina Saey, molecular biologist Anne Simon, and bioethicist Alan Regenberg. A Nobel Prize winner argues banning CRISPR babies won’t work Geneticists push for a 5-year global ban on gene-edited babies A CRISPR spin-off causes unintended typos in DNA News of the first gene-edited babies ignited a firestorm The researcher who created CRISPR twins defends...