Proteins, post-translational changes

Some special features of proteins are elaborated by secondary transformations that are not part of the translation process. The A-formylmethionine initiator may be hydrolysed to methionine, or, as we have already indicated, the methionine unit may be removed altogether. Other post-translational changes to individual amino acids may be seen, e.g. the hydroxylation of proline to hydroxyproline (see Section 13.1) or the generation of disulfide bridges between cysteine residues (see Section 13.3). 

Enzymatic modification of proteins applicable to foods is reviewed by Whitaker ( ). Described briefly are present uses of proteolytic enzymes for modifying proteins through partial hydrolysis. Major emphasis is placed on those enzymes which bring about aggregation of proteins, cross-link formation, and side chain modification through post-translational changes in the polypeptide chain. 

Table 1.2. Post-translational changes to proteins the modified coded amino acids present in proteins, including crosslinking amino acids (secondary amino acids)... 

Since iron is involved in many central nervous system processes that could affect infant behaviour and development, iron deficiency has adverse effects on brain development, both pre- and post-natal. In various epidemiological studies, it is reported that children with iron-deficiency anaemia have poorer performances on tests of some specific cognitive function. Animal experiments have identified some of the defects of reduced iron availability on brain function, which include post-translational changes (which result in a failure of iron incorporation into protein structures which are subsequently degraded), vulnerability of the developing hippocampus (with loss of the neuronal metabolic marker cytochrome c oxidase), and altered dendritic stmcture. Iron deficiency will also have a direct effect on myelin, including a decrease in myelin lipids and proteins, as well as neurotransmitter systems, since iron... 

Microsequencing provides clues about post-translational changes of the protein such as phosphorylation, sulfation, glycosylation, and the position of the disulfide bridges. Partial sequences can sometimes throw light on the relationships to other proteins. 

Post-translational modification, which extends the range of functions of the protein, is one of the later steps in the biosynthesis of many proteins. Post-translational modification of proteins occurs by methylation, hydroxylation, acetylation and phosphorylation of the protein functional groups, by attaching various lipids and carbohydrates to the protein molecule and by making structural changes such as the formation of disulfide bonds from cysteine residues by oxidation. 

While electrospray is used for molecules of all molecular masses, it has had an especially marked impact on the measurement of accurate molecular mass for proteins. Traditionally, direct measurement of molecular mass on proteins has been difficult, with the obtained values accurate to only tens or even hundreds of Daltons. The advent of electrospray means that molecular masses of 20,000 Da and more can be measured with unprecedented accuracy (Figure 40.6). This level of accuracy means that it is also possible to identify post-translational modifications of proteins (e.g., glycosylation, acetylation, methylation, hydroxylation, etc.) and to detect mass changes associated with substitution or deletion of a single amino acid. 

Post-translation modification Changes that occur to proteins after peptide-bond formation has occurred, e.g. glycosylation and acylation. 

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

Proteomics, the measurement of the global changes in proteins produced as a result of gene expression, bridges the gap between genome sequence and cellular behaviour and takes into account the post-translational modifications that often result in the functional effect. It has the potential to determine the role of protein-protein complexes in the complex signalling cascades that... 

Short-term requirements for increases in the synthesis of 5-HT can be met by post-translational processes, such as phosphorylation, that change the kinetic properties of tryptophan hydroxylase without necessitating the synthesis of more molecules of tryptophan hydroxylase. By contrast, situations requiring long-term increases in the synthesis and release of 5-HT result in the synthesis of tryptophan hydroxylase protein. For example, partial but substantial destruction (>60%) of central serotonergic neurons results in an increase in the synthesis of 5-HT in... 

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