Proteins: Their function,  their structure and deficiency disorders

Function of proteins

Protein is an important substance found in every cell in the human body. In fact, besides water, Protein the most abundant substance in the human body and has many different functions. It is used in many vital processes and thus needs to be consistently replaced by consumption of foods that contain protein. Sources of protein include meat, dairy and beans. Here are some of the functions of proteins: 

Sources of protein

• Some control  the rate of reactions and regulate cell processes   (enzymes).
• Hormones
• Structural Functions
• Some are used to form bones and muscles
• Others transport substances into or out or cells to help fight diseases
• Proteins can be used as an energy source, but this is a last resort function since proteins have many other important roles in the human body. 

Structure of proteins

Proteins are polymers of amino acids covalently linked through peptide bonds into a chain. structural features of proteins are usually described at four levels of complexity: 

• The primary structure: this level is where the specific  sequence of amino acids are located. This precised sequnce is determined by genes, and the change of one amino acid in the sequence can change the structure of the entire protein. Also, R groups of the amino acid aid in shaping the protein. - Types of bonds found: Peptide bonds (Amino acids are bonded by peptide bonds). 
• The secondary structure: this level describes the shape of the protein. There are two types of folding at this level: Alpha helices and beta pleated sheets. A protein can have a mixture of both of these types of foldings. -Types of bonds found: Hydrogen bonds (H-bonds help to stabilize the helices and pleated sheets and the type of folding is determined by the Hydrogen bonds across amino acids). 
• The tertiary structure: this level describes the overall 3-dimensional shape of proteins, which results from non-covalent interactions between amino acids. - Types of bonds/interactions found: Ionic bonds (occurs when positive R groups interact with negative R groups), Hydrophobic interactions (weak bonds occur when amino acids orient themselves away from the water, towards the interior of the structure), Hydrogen bonds (btw. polar R groups), Hydrophillic interactions (hydrophillic amino acids orient themselves towards water; outward), Disulfide bridges (strong bonds formed btw. Cysteines). 

The  quarternary structure: this level describes non-covalent interactions that bind multiple polypeptides into a single, larger protein. A protein can only make it to this level if it has more than one polypeptide chain. This level often contains a prosthetic- a non-protein atom or molecule embedded in it. For example, Hemoglobin has a quartenary structure because it has 4 polypeptide chains with 4 heme (iron) posthetic groups. - Types of bonds: Same as the tertiary structure.

Protein structure levels

Since proteins play such a major role in numerous bodily functions, a lack of proteins can lead to many disorders such as PKU and Kwarshiokor. 

1. Phenylketonuria (PKU): PKU is a genetic disorder caused by a mis-sense mutation int he gene that produces phenylalanine hydroxylase, which means that phenylalanine cannot be converted to tyrosine in the body therefore it builds up. This results in brain developmental problems and seizures. It is progressive so it must be diagnosed and treated early. Dairy, breast milk  nuts and aspartame must be avoided because they are rich in phenylalanine and will cause build up to increase.

An infant being tested for PKU.

 2. Another result of the lack of protein in the human diet is a protein deficiency disorder called  Kwashiorkor. Kwashiorkor is caused by a lack of essential amino acids in the diet. The effects of this disorder are: 

• Insufficient amount of proteins to produce membrane pumps and other protein-based structures in the body
• Fluid build up in cells (edema), as it cannot be returned back to the blood effectively due to the lack of protein transport
• Reduced growth (stunting)
• Weight loss (lack of muscle mass) 
• Developmental disability 
• Severe edema of the abdomen

  

  A child with Kwashiorkor
  
  
  

  To prevent a protein deficiency disorder adult males should consume at least 52 grams of protein a day and adult women should consume at least 45 grams per day.