Metabolic Functions

In mammals, biotin serves as an essential cofactor for five carboxylases, each of which catalyses a critical step in intermediary metabolism. All five of the mammalian carboxylases catalyze the incorporation of bicarbonate as a carboxyl group into a substrate and employ a similar catalytic mechanism.

Biotin is attached to the apocarboxylase by a condensation reaction catalyzed by holocarboxylase synthetase (Figure 1). An amide bond is formed

HN I

HC-I

H2cv o

NH I

Biocytin c=o

HN I

HC-I

HoCx

O II

NH I

t-amino group

O II

y.— (Holocarboxylase

Biotinidase^)

^CH CH2 CH2 CH2 CH2 C^

HS-CoA

y.— (Holocarboxylase

Biotin

O II

HS-CoA

HN NH

S-CoA

Biotinyl-CoA

O II

HC-CH o

Bisnorbiotin

HC-CH O

Bisnorbiotin methyl ketone

Figure 1 Biotin metabolism and degradation. Ovals denote enzymes or enzyme systems; rectangles denote biotin, intermediates, and metabolites. AMP, adenosine monophosphae; ATP, adenosine triphosphate; CoA, coenzyme A; Ppi, pyrophosphate; *, site of attachment of carboxyl moiety.

Table 1 Normal range for biotin and metabolites in human serum and urinea

Compound

Serum

Urine

(pmol/l)

(nmol/24 h)

Biotin

133-329

18-127

Bisnorbiotin

21-563

6-39

Biotin-D,L-sulfoxide

0-120

5-19

Bisnorbiotin methylketone

0-120

2-13

Biotin sulfone

ND

1-8

Biocytin

0-26

1-13

Total biotinyl compounds

294-1021b

46-128

aNormal ranges are reported (n = 15 for serum; n = 16 for urine, except biocytin, n = 10).

including unidentified biotin metabolites.

ND, not determined.

aNormal ranges are reported (n = 15 for serum; n = 16 for urine, except biocytin, n = 10).

including unidentified biotin metabolites.

ND, not determined.

between the carboxyl group of the valeric acid side chain of biotin and the "-amino group of a specific lysyl residue in the apocarboxylase; these regions contain sequences of amino acids that are highly conserved for the individual carboxylases both within and between species.

In the carboxylase reaction, the carboxyl moiety is first attached to biotin at the ureido nitrogen opposite the side chain; then the carboxyl group is transferred to the substrate. The reaction is driven by the hydrolysis of ATP to ADP and inorganic phosphate. Subsequent reactions in the pathways of the mammalian carboxylases release carbon dioxide from the product of the carboxylase reaction. Thus, these reaction sequences rearrange the substrates into more useful intermediates but do not violate the classic observation that mammalian metabolism does not result in the net fixation of carbon dioxide.

Regulation of intracellular mammalian carboxy-lase activity by biotin remains to be elucidated. However, the interaction of biotin synthesis and production of holoacetyl-CoA carboxylase in

Escherichia coli has been extensively studied. In the bacterial system, the apocarboxylase protein and biotin (as the intermediate biotinyl-AMP) act together to control the rate of biotin synthesis by direct interaction with promoter regions of the biotin operon, which in turn controls a cluster of genes that encode enzymes that catalyze the synthesis of biotin.

The five biotin-dependent mammalian carboxylases are acetyl-CoA carboxylase isoforms I and II (also known as a-ACC (EC 6.4.1.2) and p-ACC (EC 6.4.1.2)), pyruvate carboxylase (EC 6.4.1.1), methyl-crotonyl-CoA carboxylase (EC 6.4.1.4), and propio-nyl-CoA carboxylase (EC 6.4.1.3). ACC catalyzes the incorporation of bicarbonate into acetyl-CoA to form malonyl-CoA (Figure 2). There are two isoforms of ACC. Isoform I is located in the cytosol and produces malonyl-CoA, which is rate limiting in fatty acid synthesis (elongation). Isoform II is located on the outer mitochondrial membrane and controls fatty acid oxidation in mitochondria through the inhibitory effect of malonyl-CoA on fatty acid transport into mitochondria. An inactive mitochondrial form of ACC may serve as storage for biotin.

The three remaining carboxylases are mitochon-drial. Pyruvate carboxylase (PC) catalyzes the incorporation of bicarbonate into pyruvate to form oxaloacetate, an intermediate in the Krebs tricarboxylic acid cycle (Figure 2). Thus, PC catalyzes an anaplerotic reaction. In gluconeogenic tissues (i.e., liver and kidney), the oxaloacetate can be converted to glucose. Deficiency of PC is probably the cause of the lactic acidemia, central nervous system lactic acidosis, and abnormalities in glucose regulation observed in biotin deficiency and biotinidase deficiency. P-Methylcrotonyl-CoA carboxylase (MCC) catalyzes an essential step in the degradation of the branched-chain amino acid leucine (Figure 2). Deficient activity of MCC leads to metabolism of

Figure 2 Interrelationship of pathways catalyzed by biotin-dependent enzymes (shown in boxes).

3-methylcrotonyl-CoA to 3-hydroxyisovaleric acid and 3-methylcrotonylglycine by an alternate pathway. Thus, increased urinary excretion of these abnormal metabolites reflects deficient activity of MCC.

Propionyl-CoA carboxylase (PCC) catalyzes the incorporation of bicarbonate into propionyl-CoA to form methylmalonyl-CoA; methylmalonyl-CoA undergoes isomerization to succinyl-CoA and enters the tricarboxylic acid cycle (Figure 2). In a manner analogous to MCC deficiency, deficiency of PCC leads to increased urinary excretion of 3-hydroxy-propionic acid and 3-methylcitric acid.

In the normal turnover of cellular proteins, holo-carboxylases are degraded to biocytin or biotin linked to an oligopeptide containing at most a few amino acid residues (Figure 1). Biotinidase releases biotin for recycling. Genetic deficiencies of holocarboxylase synthetase and biotinidase cause the two types of multiple carboxylase deficiency that were previously designated the neonatal and juvenile forms.

Losing Weight Without Starving

Losing Weight Without Starving

Tired of Trying To Loose Weight And It Never Works or You Have To Starve Yourself Well Here's A Weight Loss Plan That takes Care of Your Weight Problem And You Can Still Eat. In This Book, You’ll Learn How To Lose Weight And Not Feel Hungry! In An Easy Step-By-Step Process That Enables You To Feel Good About Loosing Weight As Well As Feeling Good Because Your Stomach Is Still Full.

Get My Free Ebook


Post a comment