Biochemistry!

Let's take a look at the biochemistry that goes behind homocystinuria! 

Here we have the overview of the pathway:

Lets break it down and look at the pathways individually.

                    Methionine → S-adenosyl methionine → homocysteine

• Homocysteine is formed from methionine through S-adenosyl methionine 
• It is a sulfur containing amino acid 

                                           ( Cystathionine beta synthase)

                      homocysteine               →                cystathionine → cysteine

• The enzyme Cystathionine beta synthase catalyses the conversion of homocysteine to cystathionine

                                              Cysteine → methionine

• Normal circumstances, cysteine is remethylated to methionine with the help of vit B12 and folate. 
• It is necessary for the methyl group to have N5-methytetrahydrofolate
• N5-methytetrahydrofolate is supplied through vitamin B12 as methyl B12

What is the problem in the pathway that causes the disorder?

• Type 1 Homocystinuria
• Due to deficiency of cysthionine beta-synthase
• Requries pyridoxal phophate as coenzyme

                                                  ( Cystathionine beta synthase)
            Methionine →  homocysteine               →                cystathionine → cysteine

• Cysthionine beta-synthase catalyses the trans-sulphuration pathway during the catabolism of methionine to cysteine.
• When there is deficiency of the enzyme
• The step - homocysteine to cystathionine is blocked
• Leading to accumulation of homocysteine and methionine. 

Testing of blood and urine?

                                     (Urine) Homocystine  ß à homocysteine

When there is high concentration of homocysteine, there will be oxidation and it is converted to homocystine. 

The urine will then contain homocystine.

As such, testings on the blood and urine can determine if the person has homocystinuria.