Proteins › Protein Structure

Modifications of N- & C-termini

Notes

Modifications of N- & C-termini

Sections


Overview

PROTEIN MODIFICATION

  • Amino acid residues may be modified co-translationally or post-translationally
  • Amino or carboxy termini of polypeptide chain can be modified post-translationally

Limitations of this tutorial

  • We will only represent some of the many ways that amino acid residues can be modified.
  • No single protein will have the number of modifications that we show here.

N-TERMINAL MODIFICATIONS

  • N-terminal acetylation: increases protein stability (resistance to degradation)
  • Varshavsky's N-end rule: N-terminal amino acid determines protein's likelihood of being degraded --> N-terminal amino acid estimates protein half-life (N-terminal amino acid modifications introduce variability)
    N-terminal signal sequences: localization signals; protein targeted to organelle

C-TERMINAL MODIFICATION

  • GPI (glycosyl phosphatidylinositol) anchor: C-terminus glycolipid targets/anchors protein to plasma membrane
    (membrane anchors ~ most common C-term. modification)
  • C-terminal signal peptide: localization (e.g. KDEL sequence: targets polypeptide chain back to ER from Golgi)

PEPTIDE CLEAVAGE

  • Proteins synthesized in inactive precursor form (pro-proteins or pro-peptides)
  • Regulates enzyme activity (i.e. proteases)
  • Proteins that undergo peptide cleavage: digestive enzymes, fibrinogen (insoluble precursor of fibrin)

ADDITION OF HYDROCARBON AND FATTY ACID GROUPS

  • Hydrophobic isoprene & palmitoyl groups can be added to cysteine residues & myristoyl groups to amino termini --> make proteins more hydrophobic
  • 10-20 carbon atoms – proteins can be membrane-associated
    – covalent/reversible addition
  • Hydrophobic groups add to terminal amino acids – membrane anchors
  • Add to other amino acids – protein hydrophobic

Full-Length Text

  • Here we will learn about post-translational modifications of peptide residues and how they affect the structure and function of proteins.
    • Specifically, we will learn about modifications to the N-terminus and C-terminus of the polypeptide chain.

Over the next three tutorials, we will learn about a various modifications to individual amino acid residues and to the polypeptide chain that help determine the functional ability of proteins

  • Start a table.
  • Denote the following key features of proteins and protein modification.
    • Every protein has its own unique and specific amino acid sequence (the primary structure).
    • Amino acid residues may be modified during translation (co-translationally) or after translation (post-translationally).
    • The amino or carboxy terminals of a polypeptide chain can also be modified post-translationally, which (as we'll see) affects their stability and/or localization.

With those key ideas in mind, now let's draw a short polypeptide sequence.

  • Draw a short chain of amino acids, which we represent with circles with lines connecting them, which represents the polypeptide's primary structure.
  • Label the N-terminus and C-terminus, which are the beginning and end terminals of the polypeptide chain.

Before we draw these modifications, let's consider two important limitations of this particular tutorial:

  • We will only represent some of the many ways that amino acid residues can be modified, many more exist.
  • No single protein will have the number of modifications that we will show here.
  • Write that proteins may be modified on either the N-terminus or the C-terminus, which affects protein stability and/or protein localization, meaning either its half-life and/or where it localizes.

Let's first look at N-terminal modifications.

  • Draw an acetyl group on the N-terminus of our polypeptide chain.
    • Write that N-terminal acetylation increases protein stability, which makes proteins more resistant to degradation.
    • In addition, indicate that the N-terminal amino acid, itself, influences protein half-life; Varshavsky's N-end rule states that the N-terminal amino acid determines a protein's likelihood of being degraded, which allows us to use the N-terminal amino acid to estimate protein half-life (although N-terminal amino acid modifications introduce variability in half-life).
  • Draw a mailbox to show that N-terminals may also contain signal sequences, which are peptides that act as a localization signals for the protein; they allow it to be targeted to a particular organelle.
    • Think of the signal peptide sequences as the zip code for the protein since the N-terminus is the first part of the protein to be synthesized and the first part of the protein to enter the cytosol.

Now, that we have an understanding of N-terminus modification, let's look at an example of C-terminal modification, and more specifically, a modification that affects protein localization.

  • Attach a GPI anchor to the C-terminus. (GPI, which is glycophosphatidylinositol is a C-terminus glycolipid that targets and anchors the protein to the plasma membrane.
  • Write that membrane anchors are the most common C-terminal modification.
  • Like the N-terminus, the C-terminus can also have signal peptide sequences that determine protein localization, such as the KDEL sequence, which causes the polypeptide chain to be sent back to the ER from the Golgi apparatus.
  • Draw a box with a "return to sender" label to represent the KDEL signal sequence.
  • Next, draw a pair of scissors cutting the bond between two amino acids to represent peptide cleavage: part of the polypeptide chain must be cleaved in order for the protein to carry out its biological function.
  • Write that peptide cleavage, not shown here, is another terminal modification in which proteins are synthesized in an inactive precursor form, usually called pro-proteins or pro-peptides.
    • Peptide cleavage is an important way of regulating enzyme activity; proteases are excellent example of this.
    • Indicate that important examples of proteins that undergo peptide cleavage include digestive enzymes and fibrinogen (the insoluble precursor of fibrin).

Lastly, let's discuss key points regarding how large hydrocarbon and fatty acid groups make polypeptides more hydrophobic:

  • Hydrophobic isoprene and palmitoyl groups can be added to cysteine residues and myristoyl groups to amino termini to make proteins more hydrophobic.
  • These groups have 10 to 20 carbon atoms and cause proteins to be membrane-associated.
    • Their addition can be covalent or reversible.
  • Hydrophobic groups can be added to terminal amino acids to act as membrane anchors, or at other amino acids to make the overall protein hydrophobic.