A: About Protein Classification


Original Question


Answer: (e) a complete protein.




Proteins can be classified according to different criteria. There are very complex ways of doing it. Anyway, simple and common ways of classifying proteins in Biochemistry textbooks are the following:


1.- Based on the shape or role of the proteins:


a)   Globular proteins (spheroproteins):


These proteins do not aggregate. Major backbone conformation include the helix, the strand, and the turn. These proteins do metabolic work: catalysis, transport, regulation, protection…These functions require solubility in blood and other aqueous media of cells and tissues. All globular proteins are constructed with a defined interior and exterior. In aqueous solutions, hydrophobic amino acids usually are in the interior of the globular proteins, while hydrophilic ones are interacting with water.

Ex. Myoglobin and Hemoglobin.


b)   Fibrous proteins (scleroproteins) :


These proteins are water insoluble proteins with elongated shapes.  

Aggregate tightly into fibers or sheet. Most play a structural and/or mechanical role. Tend to form elongated, curvilinear structures of high regularity. The regular structure comes from high primary sequence regularity.  Usually present modified amino acids. Ex. keratin and collagen.


2.- Based on the composition:


a)     Simple Proteins: Formed only by amino acids linked through peptide bonds (polypeptide chains)


b)     Conjugate proteins: Formed by amino acids and a different compound.

In conjugate proteins, the polypeptide portion is called apoprotein and the nonpolypeptide moiety is called prosthetic groups. According to the kind of prosthetic group, conjugates proteins can be nucleoproteins, glycoproteins, hemoproteins, flavoproteins, etc.


3.- According to the Nutritional Value, proteins can be classified as:


a)   Complete: A protein that contains all the essential amino acids. Generally comes from animals.


b)   Incomplete: A protein insufficient in one or more essential amino acids. Generally comes from plants.


The only information in the question that allows us to classify the protein X is the fact that it has all the essential amino acids. It allow us to classify it as a complete protein (the lack of some non essential amino acid is not significant from the nutritional point of view, since we can synthesize non essential amino acids)


We lack information about the conformation of the protein, its function,  so we can not classify it as a fibrous or globular protein, a simple or conjugate protein, or according to its function.


Is it possible to follow a vegetarian diet and obtain all the essential amino acids in the diet?


Yes, it is possible. Besides the fact that Soy protein in fact contains the essential amino acids, the combination of incomplete proteins in the diet allow to compensate for the lack of specific amino acids in any of them, so if we combine two proteins with low nutritional value, we can obtain a mixture with a higher value that the two original proteins.This is called the  supplementary action of the proteins.


In other words, if Protein A lacks lysine and protein B lacks methionine, the combination of Proteins A+B will have a higher nutritional value than the two separate proteins. Nowadays it is considered that it is not necessary to combine incomplete proteins in the same meal.



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One thought on “A: About Protein Classification

  1. Pingback: About Protein Classification « The Biochemistry Questions Site

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