The 70 kDa Heat Shock Proteins Hsp70 have several essential functions in
living systems, such as protecting cells against protein aggregation, assisting
protein folding, remodeling protein complexes and driving the translocation
into organelles. These functions require high affinity for non-specific
amino-acid sequences that are ubiquitous in proteins. It has been recently
shown that this high affinity, called ultra-affinity, depends on a process
driven out of equilibrium by ATP hydrolysis. Here we establish the
thermodynamic bounds for ultra-affinity, and further show that the same
reaction scheme can in principle be used both to strengthen and to weaken
affinities (leading in this case to infra-affinity). We show that cofactors are
essential to achieve affinity beyond the equilibrium range. Finally, biological
implications are discussed.Comment: 14 pages, 5 figure
