*The backbone of amino acids and how to titrate them (though it is not necessary to know all the structures by heart, you should understand the nature of the R-groups in general terms such as polar/nonpolar, charged/uncharged, acidic/basic).
*The physical nature of the peptide bond.
*A general understanding of enzymes and (useful but not essential) enzyme kinetics.
*A solid understanding of the Gibbs free energy equation and equilibrium kinetics.
*A good basic understanding of carbohydrates, including the structure and formation of the anomers of the common monosaccharides (glucose, mannose, galactose, fructose) as well as the composition of the common disaccharides (sucrose, lactose, maltose) and polysaccharides (glycogen, starch, cellulose).
*An even more basic understanding of lipids, at least their most common structures and attributes.
*A general understanding of glycolysis, the Krebs cycle and the urea cycle. (You do not need to know all the individual steps, but you should know how many ATP are used/formed, how many NAD+/NADH, etc.)
You may have had some or all of this in either biology, chemistry or organic chemistry. You can get away with less, but knowing this will serve you very well on the MCAT.