Cosmology and proteins: landscape of possibilities

To the Editor

— I read Lawrence Krauss's Thesis piece “Anthropic Fever” (Nature Phys. 2, 64; 2006) with much interest. This work, along with the News and Views article “On the Shoulders of Giants” (Nature Phys. 2, 73–74; 2006), alludes to some of the controversy surrounding what is now referred to as the anthropic principle — the proposition that our Universe is one possibility among a set of different possible universes. Proponents of the anthropic principle suggest that there are many universes beyond our cosmic horizon, each having a different value for the cosmological constant. Our Universe just happens to be the lucky one with a value that is compatible with life.

As a scientist who has been interested in protein folding for quite some time, I was struck by the parallels between what is commonly referred to as the 'protein folding problem' and the anthropic principle. In his recent book¹, Leonard Susskind, a staunch proponent of the anthropic principle, mentions these similarities but, in my view, he fails to follow them to their logical conclusion.

A small protein consisting of 100 amino acids can, in principle, adopt more than 10⁴⁸ different structures. Although 10⁴⁸ may seem small relative to the number of different possible 'pocket universes' dictated by string theory (10⁵⁰⁰), the fact remains that proteins have a large number of possible conformations available to them. However, despite this diversity, most proteins fold to a unique three-dimensional structure that is only a function of the amino-acid sequence². In other words, two proteins with the same amino-acid sequence will adopt the same structure, even though both have a large number of possible structures available to them.

How is a protein able to reliably find its unique conformation among this vast set of possibilities? Within the last few decades, considerable progress has been made towards answering this question³. The solution may lie in the shape of a protein's energy landscape⁴. To give a simplistic example, suppose that the energy landscape of a given protein is shaped like a funnel in the vicinity of the 'correct' structure. If the correct conformation lies at the basin of the funnel, then all proteins with this sequence would easily, and necessarily, find their way to this unique structure. Other low-energy conformations may exist in other regions of the landscape, but if these states are kinetically inaccessible, then the probability that the protein will fold to any of these alternate conformations is very small. In some sense, it is the shape of the landscape that explains why proteins adopt a unique structure.

The point here is that there seems to be considerable information contained in the shape of a protein's landscape. Therefore, the relevant question to me is: what is it about the shape of the cosmic landscape that caused our Universe to end up in the pocket that we currently inhabit? To a physical chemist, the anthropic principle is not just a statement about diversity in the cosmic landscape, it implies something about the shape of the landscape. Moreover, the shape of the cosmic landscape is a function of the relative vacuum energies of possible universes, not chance.

Is the analogy between the cosmic landscape and a protein's energy landscape valid? Certainly, there are insufficient data at present to fully advocate such a position. However, as research about regions of the cosmic landscape near our own pocket universe evolves, we may find reasons to restore the notion of uniqueness to our Universe.