USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 17-year-old boy comes to the office because of chest pain. His mother states that the patient attends a special education program due to intellectual disability. The patient is 187.96 cm (6 ft 2 in) tall and weighs 67.1-kg (148.0-lb). Physical examination shows a slouching posture and blurry vision. An ECG shows ST changes on leads V1-V4. Which of the following proteins or enzymes is mostly likely defective, causing this patient's symptoms?
Content Reviewers:Yifan Xiao, MD
In homocystinuria, “homocysteine” is a metabolite of the amino acid methionine, and “uria” means, a substance present in urine.
So people with homocystinuria have large amounts of homocysteine in their urine, as well as other problems in the connective tissue, muscles, brain, heart, and blood vessels.
Now, amino acids are the basic building blocks that make up proteins.
There are 20 amino acids used in the human body and they all contain a carboxyl (-COOH) group and an amine (-NH2) group.
Methionine is one of the essential amino acids, meaning our bodies can't create it, but they must be acquired through food that’s rich in protein like meat, eggs, dairy, avocados, beans, etc.
So the proteins you eat are broken down into amino acids in the gastrointestinal tract by gastric acid and digestive enzymes.
Since the body can’t store these amino acids, any excess amino acids are converted into glucose or ketones and used for energy.
Now methionine is also used to synthesize another amino acid, cysteine.
First, methionine is converted into the amino acid homocysteine through multiple steps.
Next, the enzyme cystathionine beta- synthase, which requires vitamin B6 as a substrate, combines homocysteine and serine to create cystathionine.
Finally, the enzyme cystathionase converts cystathionine into cysteine.
Any homocysteine that does not undergo this process can be converted back into methionine by methionine synthase, which requires vitamin B12, or cobalamin, and folate as substrates.
There are two types of homocystinuria: familial and acquired.
Familial homocystinuria is an autosomal recessive genetic disorder that first manifests early in life.
It’s usually caused by cystathionine beta-synthase deficiency, but it can also be caused by decreased B12 affinity in cystathionine beta- synthase, or methionine synthase deficiency.
Now, when there’s a problem with cystathionine beta- synthase, homocysteine cannot be combined with serine, so less cysteine is produced, and homocysteine builds up in the body.
If methionine synthase is defective, homocysteine can be converted into cysteine, but it can’t be converted back into methionine, which also leads to its accumulation.
They also build up in the blood, where they bind to the endothelial cells lining blood vessels, causing them to secrete molecules called proinflammatory cytokines.
These attract immune cells like leukocytes to the area and cause inflammation, which leads to atherosclerosis, or plaque build up inside the arteries.
This narrows the arteries and could lead to ischemia of the tissue supplied by these arteries.
Furthermore, homocysteine also binds to platelets and cause them to stick to one another to make blood clots.
In the bones, homocysteine binds to fibrillin-1 proteins that normally provide structural support.