Protein Binds Permanently

Biological Strategy

Red junglefowl

Eleanor Banwell

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Attach Permanently

A living system can conserve energy by attaching permanently to a particular site because it can take advantage of resources that come its way, rather than expending energy to move to resources. A permanent attachment, intended to last the lifetime of the living system, creates special challenges. For example, physical mechanisms, such as the anchor that holds a marine algae to the ocean’s bottom, must be able to withstand forces that can pull it off its substrate. Chemical mechanisms, such as a barnacle’s glue, must avoid both physical and chemical breakdown, such as being dissolved by water.

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Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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Protein from chicken egg permanently binds vitamin by changing shape

Vitamin B7, or biotin, is a small molecule that’s important for cell growth. Birds, lizards and reptiles all lay eggs and the egg yolks contain everything necessary for an embryo to grow, including biotin. Biotin is also necessary for bacteria to grow, and so this makes egg yolk great food for bacteria and susceptible to infection.

One way of preventing bacterial growth is to make sure there’s no biotin available. To protect the yolk and the growing embryo, the white of eggs contains avidin, a biotin-binding that mops up any of the vitamin it comes into contact with. This makes the egg white a poor environment for bacterial growth and creates a physical protective barrier between the outside world and developing embryo.

It’s extremely important that, once it has been captured, biotin is not released back into the egg white. For this reason, the bond between avidin and biotin is one of the strongest and most stable known in nature. Although it is non-covalent and, technically, reversible, the bond is so strong that, in practice, it is permanent.

Several factors make the avidin—biotin bond so strong. Avidin contains a very deep binding pocket that is shaped perfectly to fit biotin. Once the vitamin enters the pocket there are multiple interactions that hold it in place. Small electrical charges in biotin induce corresponding electrical charges in the lining of the binding pocket. Once these charges have been induced, they act like tiny magnets, gluing the biotin in place. At the same time, there are waxy patches on the walls of the pocket that match oily parts of the smaller molecule. In the same way that oil and water don’t mix, the oily patches cannot easily move past the induced charges and they also cement biotin in place. Finally, the avidin itself changes shape following binding of biotin. Once a single molecule of biotin is inside the pocket, a portion of the avidin protein swings across the top, sealing the entrance and insuring the vitamin cannot escape.

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Last Updated July 23, 2019

References

Journal article

Structural Origins of High-Affinity Biotin Binding to Streptavidin

Science | 01/01/1989 | Weber PC, Ohlendorf DH, Wendolski JJ, Salemme FR

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Reference

Journal article

Three-dimensional structures of avidin and the avidin-biotin complex

Proc. Natl. Acad. Sci. USA. | 01/01/1993 | Livnah O, Bayer EA, Wilchek M, Sussman JL

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