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Functional Groups in Organic Chemistry

Lipids

The reactions of carboxylic acids represent one of the classes of organic chemistry reactions that will play a prominent role in biochemistry. The acyl-substitution mechanisms are especially important, appearing in one form or another all over the place in biochemistry. Fatty acids, one of the most important precursors for several significant categories of biological lipids, are long-chain carboxylic acids. Fats, phospholipids and waxes are all fatty acid esters. Most naturally occurring fatty acids are unbranched and contain an even number of carbons.



Functional Groups in Organic Chemistry

Molecular Spectroscopy

The infrared spectrograph of a typical carboxlic acid has characteristics in common with the spectrographs of both alcohols and aldehydes & ketones. The IR spectrum of a carboxylic acid shows a broad region of overlap between the O-H and C-H stretching frequencies from 2500 to 3500 cm-1. The carbonyl group gives a strong absorption at 1700 cm-1. In HNMR, the acidic proton is very strongly deshielded, appearing 10 to 12 ppm downfield (identifiable by addition of D2O).



Functional Groups in Organic Chemistry

Acids and Bases

Organic Acids and Bases

Reactions of Carboxylic Acids and Derivatives

Proteins

Amino acids are a crucially important class of carboxylic acids in biochemistry. Amino acids have a carboxyl group, amine group, hydrogen and side chain bound to a central carbon (in proline, the amine group and the side chain have also formed a ring).

From the basic fundamentals of amino acid chemistry stretching to just beyond the first footholds of biochemistry, amino acids permeate the MCAT. You really need to return to this topic many times in your MCAT preparation.

The acid-base behavior of amino acids is a special favorite MCAT topic. In neutral solution, typically the carboxyl groups are ionized to be negatively charged and amine groups are ionized to be positively charged.

In peptide bond formation, the amine group of one amino acid reacts with the carboxyl group of another in an acyl exchange mechanism, forming an amide linkage.

There may be a rare question on the MCAT that relies on you being able to identify the characteristics of a particular amino acid, such as knowing that valine is non-polar from memory, but there will not be a question at the full Biochemistry level such as expecting you to specifically distinguish valine from leucine.

Even though most students do not study the amino acids in great depth in Biology 101, you need to be aware that the AAMC is a bit disingenuous about the relationship between the test and Biochemistry. While it is true that you can definitely earn a superior score on the Biology section without having had Biochemistry, you do need to have gone a step further than Biology 101 into Biochemistry to have a good comfort level throughout the exam.

Imagine that your Biology 101 course had been taught by a Biochemistry professor, who liked to preview Biochemistry every now and then in the course, and you might be able to get an idea of level to shoot for. In summary, you don't need to panic if you have not had Biochemistry, but you do need to go a little bit further in MCAT review with your study of macromolecules and the metabolism than your basic Biology course probably took you.




Functional Groups in Organic Chemistry

Bioenergetics and Cellular Respiration

Integration of Metabolism

Acetic acid is one carboxylic acid of particularly great importance in biochemistry. Acetate derived products play many rolls in biochemistry, not only in energy metabolism but in biosynthesis. Acetic acid often takes the form of Acetyl coenzyme A in the biochemical context, which is basically acetate with a coenzyme handle on it. Acetyl coenzyme A is a thioester, derived by enzyme catalyzed oxidation and decarboxylization of pyruvic acid by Coenzyme A and NAD+. Acetyl coenzyme A is not only an important intermediate in oxidative metabolism, but it is also a critical biosynthetic precursor within amino acid and lipid synthesis.







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