[cf. et al 2018, elite education] Highly organized, universal structures underlying biological and technological networks mediate effective trade-offs among efficiency, robustness and evolvability, with predictable frailties that can be used to understand disease pathogenesis.
The aims of this article are to describe the features of one common organizational architecture in biology, the bow tie [or hourglass].
Large-scale organizational frameworks such as the bow tie are necessary starting points for higher-resolution modeling of complex biologic processes
…Bacterial metabolic networks are a striking example of ‘bow-tie’ organization and illustrate the flexibility that such a structure provides. As shown in Figure 1, a myriad of nutrient sources are catabolized, or ‘fan in’, to produce a handful of activated carriers (eg. ATP, NADH and NADPH) and 12 precursor metabolites (eg. glucose 6-phosphate, fructose 6-phosphate, phosphoenolpyruvate and pyruvate), which are then synthesized into ~70 larger building blocks (eg. amino acids, nucleotides, fatty acids and sugars). The precursors and carriers can be thought of as two ‘knots’ of separate bow ties that are both fed by catabolism, but whereas the former ‘fan out’ locally to the biosynthesis of universal building blocks, the latter fan out to the whole cell to provide energy, reducing power and small moieties.
…This robust design has inherent frailties. In a bow-tie structure, a chief source of fragility is that the universal common currencies responsible for robustness can be easily hijacked by parasites or used to amplify pathological processes. For example, tumor survival is enhanced by hijacking and upregulating processes that are part of normal physiological homeostasis. The efficiency and adaptability of metabolism, coupled with its frailties, illustrate a highly/heterogeneous optimized/organized trade-off/tolerance (HOT) architecture12; in other words, the metabolism bow-tie architecture and associated protocols allow highly optimized trade-offs between numerous requirements, such as reaction complexity (number of substrates in a reaction), genome size, efficiency (energy required for each reaction) and particularly adaptability, through tolerance to various perturbations and evolvability on longer timescales.
Some general consequences of a HOT architecture are clear. For example, if every nutrient-product combination had independent pathways without shared precursors and carriers, the total genome would be much larger and/or its encoded enzymes would be vastly more complex. In both cases, adaptation to fluctuating environments on any timescale would be difficult. Only an organization such as the bow tie facilitates the type of extreme heterogeneity that allows for robust regulation, manageable genome sizes and biochemically plausible enzymes. Bow-tie structures and protocols are found throughout biology in parallel or convergent systems, as well as in homologous systems. Furthermore, the basic framework of bow ties described here is used throughout advanced technologies. Taken together, the convergent evolution in biology and developments in technology suggest that these structures and protocols are universal.
…In the power grid, several energy sources are used to make a universal 60hz AC common carrier, which in turn is widely disseminated to provide power to a large and rapidly changing variety of uses…Perhaps the most famous technological bow tie is the Internet protocol stack ‘hourglass’ (a bow tie on its side). Here, the application layer that includes email and the Internet sits above hardware or link layers that provide raw packet delivery. Between these highly heterogeneous extremes are the universal, homogeneous, but hidden transmission control protocol/Internet protocol (TCP/IP) layers that provide routing, reliable transport and congestion control. All layers are decentralized and asynchronous, but the TCP/IP protocol suite ensures robust and coherent behavior. For example, readers accessing this article on the Internet will use various ancillary protocols at all layers of the protocol stack, including standard languages for document display (PDF and HTML). Unfortunately, the same hidden mechanisms that facilitate the transparent delivery of this article also enable the propagation of spam, viruses and denial-of-service attacks.
It is not only biology and technology that use bow-tie architectures. Money can be thought of as a common carrier that implements a bow-tie protocol for the exchange of varied goods and services. As compared with a barter system, money greatly facilitates trade and economic growth, but it increases the risk of frailties in the form of theft, counterfeiting, creative accounting and financial market collapses
[Or in cryptography, particularly in multi-party computation/zero-knowledge proofs/homomorphic encryption.]