Primary structure: linear sequence of amino acids

Secondary structure: hydrogen bonds, alpha-helices and beta-pleated sheets

Tertiary structure: disulfide bonds, single polypeptide chain

Myoglobin is a monomer, and is made of a single polypeptide chain. Thus, its highest level of protein structure is tertiary. While collagen does contain different polypeptide chains, it is an example of a protein with quaternary structure, not an explanation of what this means.

Primary structure: linear sequence of amino acids

Secondary structure: hydrogen bonds, alpha-helices and beta-pleated sheets

Tertiary structure: disulfide bonds, single polypeptide chain

Myoglobin is a monomer, and is made of a single polypeptide chain. Thus, its highest level of protein structure is tertiary. While collagen does contain different polypeptide chains, it is an example of a protein with quaternary structure, not an explanation of what this means.

Trypsin will cleave the primary sequence after the lysine residue (on its carboxyl side). Thus, Lys will be the new carboxyl terminal and Glu will be the new amino terminal. Remember that a protein's primary sequence is written from N to C.

When a protein is damaged, it can be tagged with the molecule, ubiquitin. This signals to the cell that the protein is no longer functioning properly and needs to be degraded.

Trypsin will cleave the primary sequence after the lysine residue (on its carboxyl side). Thus, Lys will be the new carboxyl terminal and Glu will be the new amino terminal. Remember that a protein's primary sequence is written from N to C.

Because an amino acid has been altered in sickle cell anemia, we can say that the amino acid sequence for hemoglobin has been changed. The amino acid sequence is defined as the primary structure for a protein, so that is the level that has been altered. It should be noted that the subsequent levels of protein structure would be altered as well, but the manipulation of the amino acid sequence is a changing of the primary structure first.

In a globular protein, the amino acid chain can twist in a way that polar groups lie at the protein's surface. This allows the protein to interact with water and enhances the protein's solubility in water. This does not occur in fibrous proteins, so fibrous proteins are insoluble in water.

To answer this question, we need to have a general understanding about amino acid properties. For instance, at physiological pH, some amino acid side chains will carry a negative charge, some will carry a positive charge, and others will be neutral. Thus, we'll need to take note of which amino acid characteristics each position has, and then evaluate each answer choice to see if the new amino acid being substituted has different characteristics.

At position is arginine, which carries a positive charge. At position is valine, which has an aliphatic side chain that is neutral and relatively hydrophobic. At position is the amino acid glutamate, which is negatively charged due to the carboxyl group on its side chain. Finally, we have glycine at position , which contains a lonely hydrogen atom as its side chain.

Now that we have the characteristics of the amino acid residues in the enzyme, let's compare them to the substitutions listed in the answer choices.

Substituting an aspartate residue into position would mean replacing valine (neutral) with a positively charged amino acid. Hence, this would likely result in disruption of enzyme activity.

Substituting a tryptophan residue into position would replace glycine. In contrast to the extremely small side chain of glycine, the side chain of tryptophan is very large. This great size discrepancy could potentially lead to steric effects that could interfere with the binding of substrate to the enzyme.

Substitution of an asparagine residue into position would replace glutamate. Because glutamate is negatively charged, whereas asparagine is neutral, this substitution would likely interfere with enzyme activity.

Finally, let's consider the substitution of arginine at position with a lysine. In this case, a positively charged arginine would be replaced by another positively charged amino acid, lysine. Because of the similarity between these two amino acids, this substitution would be the least likely to cause a disruption in the enzyme's activity.

Mammalian DNA ligase I has this function, and there are other DNA ligases which perform it in other animals and eukaryotes (prokaryotes also have their own DNA ligases). All the other functions mentioned are done by other classes of enzymes, not ligases (i.e. hydrolases, aminotransferases, oxidoreductases, etc.).

An O-linked glycoprotein is a protein that has a sugar attached to it. It is called O-linked because the sugar is attached to an oxygen atom on either a threonine residue or a serine residue within the protein.