Constructing Kinetically Controlled Denaturation Isotherms of Folded Proteins Using Denaturant-Pulse Chaperonin Binding.

Methods to assess the kinetic stability of proteins, particularly those that are aggregation prone, are very useful in establishing ligand induced stabilizing effects. Because aggregation prone proteins are by nature difficult to work with, most solution based methods are compromised by this inherent instability. Here, we describe a label-free method that examines the denaturation of immobilized proteins where the dynamic unfolded protein populations are captured and detected by chaperonin binding.

The regulatory α and ß subunits of phosphorylase kinase directly interact with its substrate, glycogen phosphorylase.

The selective phosphorylation of glycogen phosphorylase (GP) by its only known kinase, phosphorylase kinase (PhK), keeps glycogen catabolism tightly regulated. In addition to the obligatory interaction between the catalytic γ subunit of PhK and the phosphorylatable region of GP, previous studies have suggested additional sites of interaction between this kinase and its protein substrate. Using short chemical crosslinkers, we have identified direct interactions of GP with the large regulatory α and ß subunits of PhK. These newfound interactions were found to be sensitive to ligands that bind PhK.

Activation of Phosphorylase Kinase by Physiological Temperature.

In the six decades since its discovery, phosphorylase kinase (PhK) from rabbit skeletal muscle has usually been studied at 30 °C; in fact, not a single study has examined functions of PhK at a rabbit's body temperature, which is nearly 10 °C greater. Thus, we have examined aspects of the activity, regulation, and structure of PhK at temperatures between 0 and 40 °C. Between 0 and 30 °C, the activity at pH 6.8 of nonphosphorylated PhK predictably increased; however, between 30 and 40 °C, there was a dramatic jump in its activity, resulting in the nonactivated enzyme having a far greater activity at body temperature than was previously realized. This anomalous change in properties between 30 and 40 °C was observed for multiple functions, and both stimulation (by ADP and phosphorylation) and inhibition (by orthophosphate) were considerably less pronounced at 40 °C than at 30 °C. In general, the allosteric control of PhK's activity is definitely more subtle at body temperature. Changes in behavior related to activity at 40 °C and its control can be explained by the near disappearance of hysteresis at physiological temperature. In important ways, the picture of PhK that has emerged from six decades of study at temperatures of ≤30 °C does not coincide with that of the enzyme studied at physiological temperature. The probable underlying mechanism for the dramatic increase in PhK's activity between 30 and 40 °C is an abrupt change in the conformations of the regulatory ß and catalytic γ subunits between these two temperatures.

A model for activation of the hexadecameric phosphorylase kinase complex deduced from zero-length oxidative crosslinking.

Phosphorylase kinase (PhK) is a hexadecameric (αßγδ)(4) enzyme complex that upon activation by phosphorylation stimulates glycogenolysis. Due to its large size (1.3 MDa), elucidating the structural changes associated with the activation of PhK has been challenging, although phosphoactivation has been linked with an increased tendency of the enzyme's regulatory ß-subunits to self-associate. Here we report the effect of a peptide mimetic of the phosphoryltable N-termini of ß on the selective, zero-length, oxidative crosslinking of these regulatory subunits to form ß-ß dimers in the nonactivated PhK complex. This peptide stimulated ß-ß dimer formation when not phosphorylated, but was considerably less effective in its phosphorylated form. Because this peptide mimetic of ß competes with its counterpart region in the nonactivated enzyme complex in binding to the catalytic γ-subunit, we were able to formulate a structural model for the phosphoactivation of PhK. In this model, the nonactivated state of PhK is maintained by the interaction between the nonphosphorylated N-termini of ß and the regulatory C-terminal domains of the γ-subunits; phosphorylation of ß weakens this interaction, leading to activation of the γ-subunits.