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Special points of emphasis

Reactions of Organic Sulfur Compounds


Like phosphorus, sulfur takes on an added importance in MCAT organic chemistry compared to the lecture course because of the importance of sulfur containing compounds in biochemistry. This is not to say that the test will expect extensive prior knowledge of sulfur compound organic chemistry but simply that there is an increased likelihood of encountering sulfur containing compounds on the test.

One prominent example of the importance of sulfur is its presence in the side chains of two amino acids, cysteine and methionine. An MCAT favorite is to see, for example, if you know that radioisotopic 35S labeling is often used to track proteins through biological processes while 32P labeling is often used to track nucleic acids.

Cysteine contains a sulfhydryl group and methionine contains a thioether linkage. Can you picture a thioether linkage? The sulfhydryl group of cysteine is reactive. Proteins utilize cysteine residues to form disulfide bridges.

As an additional example of the importance of sulfur in biochemistry, two enzymes, thio redoxin and ribonucleotide reductase, utilize two exposed cysteine residues as a source of reducing power. Would you miss a question if you don't know that cysteine residues are used in a redox manner in biochemistry going into the MCAT? No! Could you miss a question on the MCAT if you panic when a passage includes terminology on the level of 'cysteine' or 'thio redoxin'? Yes! There will definitely be passages like that. The key is not to be intimidated. Trust your background in the fundamentals and do not let yourself be intimidated by nomenclature. Slow down and decode it. Much of the MCAT is about answering questions about basics nested within advanced contexts.

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