Proteinfaltung und Proteinassoziation

Molecular chaperones--cellular machines for protein folding

Proteins are linear polymers synthesized by ribosomes from activated amino acids. The product of this biosynthetic process is a polypeptide chain, which has to adopt the unique three-dimensional structure required for its function in the cell. In 1972, Christian Anfinsen was awarded the Nobel Prize for Chemistry for showing that this folding process is autonomous in that it does not require any additional factors or input of energy. Based on in vitro experiments with purified proteins, it was suggested that the correct three-dimensional structure can form spontaneously in vivo once the newly synthesized protein leaves the ribosome. Furthermore, proteins were assumed to maintain their native conformation until they were degraded by specific enzymes. In the last decade this view of cellular protein folding has changed considerably. It has become clear that a complicated and sophisticated machinery of proteins exists which assists protein folding and allows the functional state of proteins to be maintained under conditions in which they would normally unfold and aggregate. These proteins are collectively called molecular chaperones, because, like their human counterparts, they prevent unwanted interactions between their immature clients. In this review, we discuss the principal features of this peculiar class of proteins, their structure-function relationships, and the underlying molecular mechanisms.

Influence of serine in position i on conformation and dynamics of reverse turns

NMR spectroscopy has been employed for the conformational analysis of the cyclic hexapeptide cyclo(-D-Pro1-Ala2-Ser3(Bzl)-Trp4-Orn5(Z)-Tyr 6-) with and without protecting groups on Ser3 and Orn5. This peptide sequence was derived from the active loop sequence of the alpha-amylase inhibitor Tendamistat (HOE 467). The aim was to investigate the role of serine in position i of a standard beta-turn on the conformation and stabilization of this turn. Based on distance and torsion constraints from 2D NMR spectroscopic measurements in DMSO-d6 solution, structure refinement was accomplished by restrained molecular dynamics (MD) simulations in vacuo and in DMSO. The analysis of both structures in solution reveals a considerable effect of the unprotected serine sidechain on the adjacent beta-turn conformation. While in the protected peptide with Ser3(Bzl) a beta II-turn is observed between Trp4 and Orn5, the deprotected compound reveals a beta I-turn in this region. The beta I-turn is stabilized by a backbone-sidechain hydrogen bond from Orn5N alpha H to Ser3O gamma. Comparisons with other NMR-derived solution structures of cyclic model peptides and in some protein structures from literature reveal a general structural motif in the stabilization of beta I-turns by serine in the i position through backbone-sidechain interactions.

Inhibition of the reactivation of acid-dissociated lactate dehydrogenase isoenzymes by their aminoterminal CNBr fragments

The catalytic activity of lactate dehydrogenase isoenzymes (LDH) depends on their tetrameric structure. Stabilization of this quaternary structure is achieved by interaction of the N-terminal part of one subunit with the C-terminal region of the other subunit. The N-terminal peptides from pig M-LDH and H-LDH which are responsible for this stabilization were obtained by CNBr-fragmentation and purification on reversed-phase HPLC. The effect of these peptides on the formation of the quaternary structure of LDH-isoenzymes was investigated by monitoring the reconstitution of the catalytic activity after acid-dissociation. Low concentrations of the N-terminal peptides led to an increased, and high concentrations to a decreased yield of reconstituted LDH activity. The effects of these two peptides were isoenzyme specific. The 32 residue peptide derived from M-LDH showed the highest effect when tested with M-LDH as target enzyme but only a poor effect with H-LDH. On the other side the 33 residue peptide generated from H-LDH showed a moderate effect with both isoenzymes. The effects of the N-terminal LDH peptides are antagonized by the coenzymes NAD+ and NADH. The most significant influence was observed with NAD+ in the M-LDH peptide-M-LDH enzyme system. Comparison of the properties of the reactivation antagonists isolated from human origin with the N-terminal CNBr-peptides of LDH revealed identity in all essential properties, suggesting that the former peptides are generated by degradation of LDH.