What is Bradykinin? - Definition, Structure & Formation

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Bradykinin is a peptide found in the body, which helps control inflammation. In this lesson we will learn about the structure of bradykinin and how it is formed.

Bradykinin Definition

Nearly a third of American adults have high blood pressure, leaving them at a higher risk for stroke, heart complications, kidney damage, and many other ailments. Therefore, there is a big market for medications designed to lower high blood pressure.

Often medications are designed by looking at how the body naturally takes care of problems. Bradykinin is a peptide produced by the body to help control blood pressure and inflammation. Some blood pressure medications act by ensuring that bradykinin isn't degraded, so that it can dilate blood vessels, thus reducing blood pressure.

The reason that bradykinin isn't called a protein is because it is only made up of nine amino acids, a protein needs to have at least 20 amino acids to be considered a protein. So, bradykinin is simply called a peptide.

Bradykinin Structure

The chemical formula for bradykinin is: C50 H73 N15 O11. With a molecular weight of 1060 g/mol. This doesn't tell us much about the structure does it? Let's look to its amino acids then.

The amino acid sequence of bradykinin is:

  • Arginine
  • Proline
  • Proline
  • Glycine
  • Phenylalanine
  • Serine
  • Proline
  • Phenylalanine
  • Arginine

Which is abbreviated as: Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg. Or, with one letter abbreviations: RPPGFSPFR

The structure of bradykinin

The two arginine residues makes bradykinin fairly basic, since arginine has the highest pKa out of all of the amino acids. The pKa refers to concentration of hydrogen ions in a solution. The higher the pKa the lower the concentration of hydrogen ions, so the higher the pKa the weaker the acid (or the stronger the base).

All of the other residues are fairly neutral, which further drives up the isoelectric point of bradykinin. The isoelectric point refers to the point when the protein, peptide, or amino acid is neutral overall. In other words, it is the point when there may be 1 positive charge, but there is 1 other negative charge to balance the entire molecule out to neutral.

Since this is only a peptide, and not a protein, it isn't big enough to have any intermolecular interactions, instead it only interacts with receptor proteins.

Bradykinin Formation

Bradykinin probably starts out as a plasma protein, specifically alpha-2 globulin. A proteolytic cleavage of the plasma protein forms a high-molecular weight kininogen (HMWK), which is a plasma protein that helps blood coagulate. Then the enzyme kallikrein can convert the HMWK into bradykinin.

The HMWK can be converted into kallidin (another peptide used for helping to heal injuries) or into bradykinin. But even if the HMWK is converted into kallidin, another enzyme, an aminopeptidase, can be used to convert the kallidin into bradykinin. This is because kallidin is almost exactly like bradykinin, except on the nitrogen-terminal a lysine is added. So by removing this lysine, bradykinin is formed.

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