12.3 Other Water Soluble Vitamins

12.3.1 Vitamin B₆

Figure 12.15: Vitamin B6s

The vitamin B₆ family consists of three closely-related molecules that only differ in their oxidation states or the amination of the carbon atom on the fourth position of the pyridine ring (see figure 12.15).

Figure 12.16: PLP prosthetic Group

Pyridoxal 5’-phosphate (i.e., PLP - see figure 12.16) is a prosthetic group for many enzymes that catalyze a variety of reactions:

1. Transanimation
2. Decarboxylation
3. Racemization

12.3.1.1 Reactions of vitamin B₆

Figure 12.17: Aldimines

In all PLP-dependent enzymes, the carbonyl group of the prosthetic group is bound as an imine to the \(\epsilon\)-amino group of a lysine residue at the active site (see figure 12.17). This enzyme-coenzyme Schiff base is sometimes called an internal aldimine.

Nonetheless, in the first step of all PLP-dependent reactions, PLP is linked to the \(\alpha\)-amino group of the amino acid (i.e., the external aldimine).

12.3.2 Vitamin B₁₂

Figure 12.18: Structure of Vitamin B12

The structure of this vitamin (see figure 12.18) includes a corrine ring that resembles the porphyrin ring system of heme.

The vitamin is present in isomerization reactions (i.e., the shifting of hydrogen atoms between carbons). Vitamin B₁₂ is also capable of acting as a methyl group carrier (i.e., it can accept methyl groups from tetrahydrofolate derivatives).

Pernicious anemia is a potentially fatal condition caused by a cobalamin deficiency. Here, there is a decreased production of blood cells by the bone marrow.

12.3.3 Vitamin H

Figure 12.19: Structure of Vitamin H

Vitamin H (i.e., biotin - see figure 12.19) is a prosthetic group for enzymes that catalyze carboxyl group transfer reactions and ATP-dependent carboxylation reactions.

It is also a specialized carrier of one-carbon groups in their most oxidized form (i.e., CO₂).

12.3.4 Vitamin B₉

Figure 12.20: Structure of Vitamin B9

This is also known as folic acid. It has three major components (see figure 12.20):

1. Pterin (i.e., 2-amino-4-oxopteridine; in black)
2. p-aminobenzoic acid (i.e., PABA; in pink)
3. Gluatamate residue (in blue)

Figure 12.21: Structure of Tetrahydrofolate

Tetrahydrofolate - the coenzyme form of vitamin B9 - differs from vitamin B₉ in two ways (see figure 12.21):

1. Tetrahydrofolate is a reduced compound.
2. It is modified via the addition of glutatmate residues bound to one another through \(\gamma\)-glutamyl amide linkages.

12.3.5 Vitamin B₁

Figure 12.22: Structure of Vitamin B1

This is also known as thiamine (see figure 12.22). The vitamin has a pyrimidine ring and a positively charged thiazolium ring.

Figure 12.23: Structure of Thiamine Pyrophosphate

The coenzyme form of vitamin B₁ thiamine pyrophosphate is used in ATP-dependent phosphorylation.

12.3.6 Vitamin B₅

Figure 12.24: Structure of Vitamin B5

This is also known as pantothenic acid (see figure 12.24).

Figure 12.25: Structure of Coenzyme A

This vitamin is a precursor of coenzyme A (see figure 12.25) and the prosthetic group acyl carrier protein (i.e., ACP) domain (this is present in fatty acid synthase).

12.3.7 Other metabolic pathways involving vitamin Bs

Figure 12.26: Other Pathways that Utilize Vitamin Bs

Figure 12.26 presents several more pathways that utilize vitamin Bs as electron carriers or cofactors.