[Technicalhuman]

Do fibrous proteins have a fibrous structure or do they only have a secondary structure? Checking many sites it seems to suggest both things but I'm not too sure about it. Firstly fibrous proteins consists of either the alpha helix or the beta structures which are secondary structures. But secondly its me tipped that all proteins have a unique 3D structure which is obtained at the secondary structure already. So I'm thinking that fribrous proteins only have up to the secondary structure.

But what if more than 1 peptide chains come together in a fibrous protein. If they only have a secondary structure, now can they jump to a quaternary structure like collagen. So I'm thinking fibrous proteins should also have a tertiary structure? But I'm not sure about that because in order to have a tertiary structure, they must have other side chain interactions like ionic bonds which the fibrous proteins don't have I think.

Then for coenzymes, are metal ions a prosthetic group or coenzyme? Because in some sources prosthetic group is just a tightly bound non protein component. While in other, it's a covalently bonded non protein group. So I'm not sure where to classify it actually.

Thanks for the help.

[Babcock_Hall]

Cofactors are coenzymes (organic) or metal ions, that help the enzyme catalyze a reaction.  Not everyone uses the term prosthetic group identically.  I use the definition provided by Nelson and Cox, which is essentially that it is any nonprotein group that is required for a protein (not necessarily an enzyme) do its job.

The defining qualities for fibrous proteins is that they are insoluble and that they are structural.  But different fibrous proteins have different secondary.  Collagen has a triple helix for much of its secondary structure, alpha-keratin (hair) has a particular kind of alpha helix.  Beta-keratin (silk) has layers of beta-sheets.  What makes a fibrous protein a little different from other proteins is that the same secondary structural element is often repeated many times.

Do you know what the definition of tertiary structure is?

[Technicalhuman]

Cofactors are coenzymes (organic) or metal ions, that help the enzyme catalyze a reaction.  Not everyone uses the term prosthetic group identically.  I use the definition provided by Nelson and Cox, which is essentially that it is any nonprotein group that is required for a protein (not necessarily an enzyme) do its job.

The defining qualities for fibrous proteins is that they are insoluble and that they are structural.  But different fibrous proteins have different secondary.  Collagen has a triple helix for much of its secondary structure, alpha-keratin (hair) has a particular kind of alpha helix.  Beta-keratin (silk) has layers of beta-sheets.  What makes a fibrous protein a little different from other proteins is that the same secondary structural element is often repeated many times.

Do you know what the definition of tertiary structure is?

I think a tertiary structure is when many different interactions cause the protein to fold into a unique 3D structure while secondary structures only include the alpha helices and beta structures (beta sheets and beta turn) and many other side chain interactions. But I'm not sure about whether fibrous proteins actually has a tertiary structure as in my notes they only say that they are built of repetitive structures. What is the case here actually?

Thanks for the help.

[Babcock_Hall]

As long as a protein has a well defined three-dimensional structure, it has tertiary structure, which is the case here.  The fact that a particular secondary structure repeats in a fibrous protein is only important in defining what the tertiary structure is.  Consider alpha-keratin versus silk fibroin versus collagen.  Look at drawings of each.  Do they have a three-dimensional structure? 

I would not define secondary structure in the way that you did.  I would define it as a portion of polypeptide that has a regular pattern of hydrogen bonding and in which all of the φ and ψ dihedral angles are similar in value to each other.  Examples include the alpha-helix, the beta-conformation