A Conversation on Protein Folding

Structural disorder originates beyond narrow stoichiometric margins of amino acids in naturally occurring folded proteins

Rigorous analyses of Euclidean distances between non-peptide bonded residues in structures of several thousand naturally occurring folded proteins yielded a surprising "margin of life" for percentage occurrence of individual amino acids in naturally occurring folded proteins. On one hand, the concept of "margin of life", referring to lower than expected variances in average stoichiometric occurrences of individual amino acids in folded proteins, remains unchallenged since its discovery a decade ago. On the other hand, within this past decade there has been a strong emergence of a gradual paradigm shift in biology, from sequence-structure-function in proteins to sequence-disorder-function, fuelled by discoveries on functional implications of intrinsically disordered proteins (primary sequences that do not form stable structures). Thus the applicability of "margin of life" to peptide-bonded residues in all known natural proteins, adopting stable structures vis-à-vis intrinsically disordered needs to be explored. Therefore in this work, we analyze compositions of the complete naturally occurring primary sequence space (over 560000 sequences) after dividing it into mutually exclusive subsets of structured and intrinsically disordered proteins along with a subset without any structural information. While finding that occurrence of different peptides (up to pentapeptides) is a direct consequence of the relative occurrences of their constituting residues in folded proteins, we report that structural disorder in natural proteins originates beyond the narrow stoichiometric margins of amino acids found in structured proteins.Communicated by Ramaswamy H. Sarma.

Homology modeling and docking studies of FabH (β-ketoacyl-ACP synthase III) enzyme involved in type II fatty acid biosynthesis of Chlorella variabilis: a potential algal feedstock for biofuel production

The concept of using microalgae as an alternative renewable source of biofuel has gained much importance in recent years. However, its commercial feasibility is still an area of concern for researchers. Unraveling the fatty acid metabolic pathway and understanding structural features of various key enzymes regulating the process will provide valuable insights to target microalgae for augmented oil content. FabH (β-ketoacyl-acyl carrier protein synthase; KAS III) is a condensing enzyme catalyzing the initial elongation step of type II fatty acid biosynthetic process and acyl carrier protein (ACP) facilitates the shuttling of the fatty acyl intermediates to the active site of the respective enzymes in the pathway. In the present study, a reliable three-dimensional structure of FabH from Chlorella variabilis, an oleaginous green microalga was modeled and subsequently the key residues involved in substrate binding were determined by employing protein-protein docking and molecular dynamics (MD) simulation protocols. The FabH-ACP complex having the lowest docking energy score showed the binding of ACP to the electropositive FabH surface with strong hydrogen bond interactions. The MD simulation results indicated that the substrate-complexed FabH adopted a more stable conformation than the free enzyme. Further, the FabH structure retained its stability throughout the simulation although noticeable displacements were observed in the loop regions. Molecular simulation studies suggested the importance of crucial hydrogen bonding of the conserved Arg(91) of FabH with Glu(53) and Asp(56) of ACP for exhibiting high affinity between the enzyme and substrate. The molecular modeling results are consistent with available experimental results on the flexibility of FabH and the present study provides first in silico insights into the structural and dynamical aspect of catalytic mechanism of FabH, which could be used for further site-specific mutagenic experiments to develop engineered high oil-yielding microalgal strains for biofuel production.