John T. Moore - Biochemistry For Dummies

An acid may be capable of donating more than one hydrogen ion. A biologically important example of this type of acid is phosphoric acid (H 3PO 4), which is a triprotic acid (meaning it can donate three hydrogen ions, one at a time). The equilibria for this acid are

The subscripts are modified to indicate the loss of hydrogen 1, hydrogen 2, or hydrogen 3. The associated K aexpressions are all of the form

Here’s the breakdown for each K a:

The value for each successive equilibrium constant often is significantly lower than the preceding value. Table 2-2illustrates some biologically important acids. You can refer to this table when working buffer problems or determining which acid is stronger.

TABLE 2-2The K aValues for Biologically Important Acids

Acid	Ka1	Ka2	Ka3
Acetic acid (CH 3COOH)
Pyruvic acid (CH 3COCOOH)
Lactic acid (CH 3CHOHCOOH)
Succinic acid (HOOCCH 2CH 2COOH)
Carbonic acid (H 2CO 3)
Phosphoric acid (H 3PO 4)

Some substances can’t make up their minds about what they are; they can act as either an acid or a base. Chemists classify these substances as amphiprotic or amphoteric substances. For example, the bicarbonate ion can act as either an acid or a base:

Biochemically important molecules may also exhibit amphiprotic behavior. Amino acids contain both a basic amine group and an acidic carboxyl group. Therefore, they can act as either acids or bases. For example, glycine may undergo the following reactions:

In fact, amino acids may undergo proton transfer from the carboxyl end to the amine end, forming an overall neutral species that has a positive and negative end. Species such as these are called zwitterions (not to be confused with twitterions — people who tweet until their thumbs fall off):

A buffer solution contains the conjugate acid-base pair of any weak acid or base in relative proportions to resist pH change when small amounts of either an acid or a base are added. Therefore, buffers control the pH of the solution. Buffer solutions are important in most biological systems. Many biological processes proceed effectively only within a limited pH range. The presence of buffer systems keeps the pH within this limited range.

In the human body, the pH of various body fluids is important. The pH of blood is 7.4, the pH of stomach acid is , and the pH in the intestinal tract is . If the pH of blood is more than 0.2 pH units lower than normal, a condition known as acidosis results; a corresponding increase in pH of about the same magnitude is alkalosis. Acidosis and alkalosis, which may lead to serious health problems, each have two general causes:

Respiratory acidosis is the result of many diseases that impair respiration, including pneumonia, emphysema, and asthma. These diseases are marked by inefficient expulsion of carbon dioxide, leading to an increase in the concentration of carbonic acid, H2CO3.

Metabolic acidosis is due to a decrease in the concentration of (the bicarbonate ion). This decrease may be the result of certain kidney diseases, uncontrolled diabetes, and cases of vomiting involving nonacid fluids. Poisoning by an acid salt may also lead to metabolic acidosis.

Respiratory alkalosis may result from hyperventilation because this excessive removal of carbon dioxide can lead to a decrease in the H2CO3 concentration. Immediate treatment includes breathing into a paper bag, which increases the carbon dioxide concentration in the inhaled air and, therefore, in the blood.

Metabolic alkalosis may result from excessive vomiting of stomach acid.