Sulfur Biochemistry of Garlic: the Biosynthesis of Flavor Precursors
H.A. Collin, J. Hughes, A. Tregova, J.G.C. Milne, G. van der Werff, M. Wilkinson,
M.G. Jones and A.B. Tomsett
The School of Biological Sciences, The Biosciences Building, Crown Street, The
University of Liverpool, Liverpool L69 7ZB United Kingdom
R. Cosstick
The Department of Chemistry, Robert Robinson Laboratories, The University of
Liverpool, Liverpool L69 7ZD, United Kingdom
L. Trueman, T. Crowther, L. Brown and B Thomas
HRI Wellesbourne, Wellesbourne, Warwick, CV35 9EF, United Kingdom
Keywords: Allium sativum, alliin, cysteine synthase, sulfur metabolism, -glutamyl
cysteine sulfoxide
Abstract
The major flavour precursors in the genus Allium include alliin and isoalliin,
members of the alk(en)yl cysteine sulfoxide group (CSOs). There are also large
amounts of S-alk(en)yl gamma-glutamyl-cysteine sulfoxide compounds. It has been
suggested that alliin is derived from serine and an allyl thiol source, or from
glutathione and an allyl source, while isoalliin is synthesised via glutathione and a
series of gamma-glutamyl peptides. We have combined several approaches to
investigate the intermediates and enzymes involved in the biosynthetic pathways.
These include profiling and tracking sulfur components in garlic leaf, clove and
root tissue as bulbs develop in the greenhouse. This has allowed us to build up, for
the first time, a picture of the dynamic uptake, allocation and distribution of
flavour precursors during the life cycle. This indicated that CSOs are remobilised
to the new cloves in a late stage of development, following uptake of the bulk of
sulfur by the plant. Experiments involving feeding potential intermediates to onion
and garlic callus tissue, that generally lack flavour precursors, indicated that
several alk(en)yl thiols could be converted to alk(en)yl cysteine and alk(en)yl
cysteine sulfoxide by callus. This may indicate that synthase and oxidase enzyme(s)
with broad substrate specificity may exist in Allium. Using data derived from the
scientific literature, we have searched for genes and enzymes that may be involved
in the biosynthetic pathway. Cysteine synthase (CS) and serine acetyltransferase
(SAT) may play a role in flavour precursor biosynthesis. Sequences with significant
homology to two groups of plant CS and to a SAT have been obtained from garlic
(cultivar Printanor). The sequence of one further CS has been identified, using
partial peptide sequences from a protein that showed allyl cysteine synthase
activity. Our results provide further insight into the biosynthetic pathway of garlic
flavour compounds.
INTRODUCTION
Garlic (Allium sativum L.) has long been valued for adding flavour to food and
health benefits are also attributed to it (Kik et al., 2001). Both originate in damaged
tissues from enzymatic cleavage of pre-formed stable flavour precursors by alliinase.
The flavour precursors are secondary metabolites, the alk(en)yl cysteine sulfoxides
(CSOs), primarily allyl cysteine sulfoxide (ACSO, alliin, 2-propenyl cysteine sulfoxide)
and methyl cysteine sulfoxide (MCSO, methiin). The synthesis and storage of these
compounds, and related -glutamyl cysteine sulfoxide peptides (-GP), results in garlic
containing the highest amount of sulfur recorded in any plant species (Nielsen et al.,
1991). Garlic sulfur compounds have shown potential in combating cardiovascular
disease and some cancers (Kik et al., 2001), and there is therefore interest in improving
the content of these compounds in garlic.
However, understanding of the biosynthetic pathway is minimal, especially when
compared with current knowledge of the assimilation of sulfur from sulfate to cysteine
(Leustek, 2000). Two routes have been proposed, one via glutathione (Lancaster and
Shaw, 1989) and one via serine (Granroth, 1970) as illustrated in Fig. 1. For both
pathways, some intermediates and the majority of genes and enzymes remain to be
identified. It is possible that each pathway dominates at a different time in the life-cycle
or in different organs. During its biennial life-cycle the bulb of garlic cloves develops
from axillary buds during late summer. These act as over-wintering storage organs, with
re-use of stored carbon, nitrogen and sulfur resources from CSOs and -GPs as roots and
leaves develop at the start of the growing season. During active growth, fresh sulfur is
assimilated from the soil via the roots. There is evidence that CSO synthesis occurs in
the leaf, with products being translocated to the developing bulb towards the end of the
growing season (Lancaster et al., 1986). The cloves also contain substantial amounts of
-GPs. The content and relationship of CSOs and -GPs are thus the result of
biosynthesis and resource allocation.
A greater understanding of sulfur partition during the life cycle and the
biosynthetic pathways for CSOs and -GPs would aid development of varieties and
agronomic procedures to yield high quality garlic with an improved content of flavour
precursors. To this end, we have reviewed information in the scientific literature for
CSO synthesis in Allium and other species and initiated research, focusing on the role of
cysteine synthases.
CSO SYNTHESIS IN ALLIUMS
Experimental studies on flavour precursor biosynthesis in onion (Allium cepa L.)
and garlic leaves, roots and tissue cultures, summarised in Table 1, shows that the
addition of exogenous thiols or alk(en)yl cysteines can give rise to the corresponding
alk(en)yl cysteines or alk(en)yl cysteine sulfoxides. In several studies, radiolabelled
precursors were used and indicated quantitative conversion to radiolabelled products
(Parry and Sood, 1989; Ohsumi et al., 1993). In addition, these studies indicated that
onion roots and both onion and garlic callus could synthesise a wide range of alk(en)yl
cysteines and alk(en)yl cysteine sulfoxides when provided with suitable precursors. For
example, provision of benzenemethane thiol and ethane thiol result in synthesis of benzyl
cysteine and ethyl cysteine, respectively (Granroth, 1970; Prince et al., 1997). The
former has not been detected within Allium sp. while the latter has only been
occasionally reported in trace amounts (Kubec et al., 2000).
Flavour precursors are normally absent from callus, or present at only trace
levels, indicating that biosynthetic capacity is not complete but the studies summarised in
Table 1 indicate that synthesis is possible with provision of suitable substrates. An
advantage of callus is that exogenous substrates can enter the intact tissue through the
thin cuticle that develops in the humid culture conditions. This avoids any problems
with cleavage of flavour precursors by alliinase that might occur in damaged tissue.
This experimental information therefore, although not excluding the glutathione
pathway, supports the serine biosynthetic route as illustrated in Fig. 1B.
CSO SYNTHESIS IN OTHER PLANT SPECIES
Synthesis of MCSO is not confined to Allium sp., but also occurs in the
Brassicaceae. Additionally, other CSOs have been reported in several unrelated tropical
plants and within fungi. For example, roots of the perennial Amazonian shrub garlic
weed (Petiveria alliacea, Phytolaccaceae) contain benzyl cysteine sulfoxide and several
species of the basidiomycete Marasmius contain methylthiomethyl cysteine-4-oxide
(marasmine) (see Jones et al., 2004 for a summary of these reports). The implication is
that this secondary metabolic capacity may have evolved on more than one occasion, and
that Allium sp. are only one, particularly intensively studied, example.
Secondary products are frequently synthesised by pathways with parallels in
primary metabolism, following gene duplication and mutation. Reports of CSO
synthesis by primary metabolic enzymes are thus particularly interesting. Ikegami and
Murakoshi (1994) reviewed the synthesis of several non-protein -substituted alanines
and some higher homologues in plants, indicating that cysteine synthase is the key
biosynthetic enzyme. Alk(en)yl cysteines are members of this class of compound and
could be synthesised, in vitro, by cysteine synthase isolated from a surprisingly wide
range of plants. Before considering the significance of this finding for flavour precursor
biosynthesis in garlic, a description of the role of cysteine synthase in primary cell
metabolism is necessary.
CYSTEINE SYNTHESIS
The synthesis of the aminoacid cysteine is catalysed by two consecutive
enzymes, serine acetyltransferase (SAT, EC 2.3.1.30) and cysteine synthase (CS, also
known as O-acetylserine thiol lyase (EC 4.2.99.8)) (see Fig. 2.). The former activates
serine to the intermediary compound O-acetyl serine (OAS), while the latter catalyses the
formation of cysteine through insertion of sulfide into O-acetyl serine (Saito, 2000). The
activity of SAT is the rate-limiting step in cysteine synthesis in planta. This enzyme is
catalytically active when in a heteromeric complex with CS, although the OAS
synthesised in the complex is released to be utilised by free CS for cysteine synthesis.
The overall reaction is a key step in sulfur assimilation in plants, since cysteine is the
first organic sulfur compound formed within the cell, uniting carbon, nitrogen and sulfur
metabolism. The process is tightly regulated at both transcriptional and posttranslational levels, the latter operating through protein-protein interactions, sub-cellular
compartmentation and modulation by substrates and products (Saito et al., 2000). At
least three isoforms of CS and SAT have been identified in plants, located in the cytosol,
chloroplast and mitochondrion, although the need for cysteine synthesis in each
compartment is not clear. CS contains a pyridoxal 5’-phosphate co-factor and has now
been classified within the subfamily of -substituted alanine synthase in the large
superfamily of pyridoxal-phosphate enzymes (Hatzfeld et al., 2000)
Despite the obvious importance of cysteine synthesis in plant primary
metabolism, there is substantial evidence that some CSs have evolved additional
biochemical properties. Activity as a cyanoalanine synthase has been attributed to the
mitochondrial CS isoforms from spinach (Spinacia oleracea L.) and Arabidopsis
thaliana (Hatzfeld et al., 2000). Mitochondria in some plants can detoxify cyanide,
produced during ethylene biosynthesis, through synthesis of cyanoalanine from cysteine
and cyanide, liberating sulfide. Although recombinant cytosolic and chloroplastic forms
of spinach CS could also synthesise cyanoalanine, they displayed low cyanoalanine
synthase activity relative to the rates of cysteine synthesis. This contrasted with the
mitochondrial isoform, which was more active in cyanoalanine than cysteine synthesis.
Measurement of Km values showed that the mitochondrial isoforms had a much higher
affinity for cyanide than sulfide, and the reverse was true for the cytosolic and
chloroplastic forms (Hatzfeld et al., 2000).
Although the relative ratio of these two reactions in vivo is likely to be controlled
by the availability of sulfide and cyanide, structural changes in the enzymes may also
affect substrate usage. Further kinetic and protein sequence analysis showed that a
single aminoacid substitution in the substrate-binding pocket of the enzyme may hinder
OAS binding (Warrilow and Hawkesford, 2002) and that the major mechanistic
difference between CS and cyanoalanine synthase probably lies in the binding and
processing of the sulfide or cyanide anions.
CYSTEINE SYNTHASES IN SECONDARY METABOLISM
There is substantial evidence that some CSs are active in secondary metabolism.
Early studies on cysteine synthesis in plants indicated that thiols, as well as sulphide,
were substrates in vitro (e.g. Thompson and Moore, 1968). Heterocyclic substituted
alanines are present in several plants and are of interest because of their pharmacological
and toxic properties. Examples include mimosine from members of the Leguminosae
which has thyrotoxic and depilatory properties and -(isoxazolin-5-on-2-yl)-alanine from
Pisum sp. and Lathyrus sp. which has antifungal activity (Ikegami and Murakoshi,
1994). These are synthesised by condensation of an appropriate N-heterocyclic
compound with OAS. Initial studies on the biosynthetic enzyme from two plants
indicated that both required OAS but had different specificities for the heterocyclic
acceptor. Tests of a range of acceptors showed that both enzymes could catalyse the
synthesis of methyl cysteine in addition to the natural product. Purification of several
enzymes for -substituted alanine synthesis indicated that several of them co-purified
with peaks in CS activity (Ikegami et al., 1990).
This circumstantial evidence for the identity of the -substituted alanine
biosynthetic enzymes as a type of CS was strengthened by studies of the
physicochemical and kinetic properties of two isoenzymes from eight species that had
been purified to apparent homogeneity. The species included model plants such as
spinach (Spinacia oleracea) and pea (Pisum sativum) as well as species that produced
unusual -substituted alanines such as rangoon creeper (Quisqualis indica,
Combretaceae). The physical properties of the enzymes were within the range typical of
CS (Ikegami and Murakoshi, 1994). In a standard assay system, OAS was the sole
alanyl donor and each enzyme demonstrated a distinct substrate specificity against a
panel of N-heterocyclic and thiol compounds, relative to synthesis of cysteine. All could
form methyl cysteine, allyl cysteine and carboxymethyl cysteine from OAS and the
appropriate thiol, but at very different rates. In most, rates were only a few per cent of
that for cysteine synthesis.
Interestingly, CS isoenzyme B from the leadtree (Leucaena leucocephala,
Fabaceae), a tree forage legume native to tropical America, could synthesis allyl cysteine
at a rate 73.2% of that for cysteine, while isoenzyme A could synthesise methyl cysteine
and allyl cysteine at 29.6% and 21.6% respectively. Isoenzyme B from watermelon
(Citrullus vulgaris, Cucurbitaceae) was also able to synthesise methyl cysteine at 71.9%
that of cysteine. The natural -substituted alanines synthesised in these two plants are
the N-heterocyclic compounds mimosine and -(pyrazol –1-yl) alanine. Only CS from
leadtree and one other of the eight plants could synthesise mimosine, although the rate
for even the most effective isoenzyme B from leadtree was only 15.7% that for cysteine.
Although isoenzymes from all eight plants could synthesise -(pyrazol –1-yl) alanine,
the rates were generally less than 2% of the rate of cysteine synthesis, including the
watermelon CSs (Ikegami et al., 1990; Ikegami and Murakoshi, 1994).
It is therefore clear that CSs from many plants are capable of synthesising
alk(en)yl cysteines when provided with a suitable substrate. The relationship between
biosynthetic rates in vitro and substrate specificity in vivo is complex, since in these
experiments biosynthetic rates with substrates that would not be encountered in vivo
were sometimes higher than for possibly natural substrates.
CYSTEINE SYNTHESIS IN ALLIUMS
Comparatively little is known about the genes and enzymes involved in cysteine,
CSO or -GP biosynthesis in Allium. The genus may have a greater requirement for
cysteine than most plants to support synthesis of the secondary metabolites that may lead
to a higher flux and/or intracellular levels. Measurements of total cysteine content in
chinese chives (Allium tuberosum) indicated that it was five- to six-fold higher than in
Arabidopsis (Urano et al., 2000). Cysteine affects the activity of SAT, which is the ratelimiting step in cysteine synthesis in planta. It has been established that some SATs are
inhibited by physiological micromolar levels of cysteine in an allosteric manner (e.g.
watermelon, cytosolic Arabidopsis, chloroplastic pea and spinach) while others (e.g.
chloroplastic Arabidopsis) are not (Droux, 2003).
The cDNA sequences have been obtained for a cytosolic SAT and CS from
chinese chive (Urano et al., 2000). Both were expressed constitutively in leaves and
roots. Expression of this SAT in Escherichia coli yielded an enzyme activity that was
inhibited by cysteine (IC50) at the comparatively high concentration of 48.7 M, about
ten-fold higher than typical inhibitory levels. Although tissue levels of cysteine in
chinese chives are higher than in some other plants, further work would be required to
elucidate whether this difference in IC50 is significant for CSO and -GP synthesis.
Two forms of CS have been purified from chinese chive leaves (Ikegami et al.,
1993) which displayed similar physical and kinetic properties to CSs from other plants,
and were able to synthesise methyl-, allyl- and carboxymethyl- cysteine in vitro when
provided with suitable thiol substrates. We have obtained sequences for three distinct
CSs from garlic which vary in their tissue expression and ability to synthesise allyl
cysteine (Tregova et al., 2004).
PROSPECTS FOR A MOLECULAR UNDERSTANDING OF CSO SYNTHESIS
Research into flavour precursor biosynthesis in Allium is at a stage where
developments in understanding of cysteine and glutathione metabolism over the last few
decades can be applied with the aim of increasing CSO synthesis. We have focused on
the role of CS, since a role for this enzyme in the synthesis of -substituted alanines has
been demonstrated. It is clear that CSs from many plants are able to synthesise alk(en)yl
cysteine when provided with suitable substrates. However, more detailed kinetic studies
of recombinant Allium CSs from each cell compartment are needed to provide a better
appreciation of their biosynthetic potential.
The importance of CS in CSO synthesis in vivo in Allium sp. remains to be
determined. Over-expression or silencing of endogenous CS genes might provide an
answer, although the effect of transformation of other plant species with genes for SAT
and/or CS has had varying results. Some report little effect on phenotype while others
have demonstrated plants exhibiting elevated levels of OAS, cysteine, glutathione or
other metabolites (Sirko et al., 2004). Since cysteine synthesis is regulated at multiple
levels in vivo this variety of effects is not unexpected and careful analysis of transgenic
phenotypes would be required. The nature of the genes and enzymes required for
sulfoxidation of the S-alk(en)yl group remains to be determined, as well as those
required for the proposed glutathione pathway. Their identification is important to
obtain the full picture of CSO biosynthesis and to explore the relationships between the
glutathione and serine pathways.
Additional information on sulphur resource allocation during growth and bulbing
would assist in improving agricultural systems for garlic production, and could be used,
in conjunction with knowledge about the biosynthetic pathways, to improve garlic
varieties through selection among existing cultivars. Although garlic can only be
propagated vegetatively at present, if fertile varieties became available these aspects of
sulfur biochemistry might provide markers to assist breeders. If transgenic garlic were to
be developed as a commercial crop, these studies could determine whether targeting
transgenes to particular organelles, tissues or times during development would be
beneficial in influencing CSO synthesis.
ACKNOWLEDGEMENTS
We would like to thank all our EU Garlic and Health project partners for their
comments and advice over the past four years. This research was financed by an EU FP5
grant in the area of Key Action 1 (QLK1-CT-1999-498).
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Tables
Table 1. The production of substituted cysteines and cysteine sulfoxides in onion and
garlic tissues after supplementation with organic thiols or substituted cysteines.
Tissue
Onion leaf
Onion bulb
Onion callus
Onion root
cultures
Garlic leaves
Differentiating
garlic callus
Garlic callus
Substrate
Methane thiol
Ethane thiol
Propyl thiol
Allyl thiol
Benzenemethanethiol
2-Thioethanol
Product
Methyl cysteine
Ethyl cysteine
Propyl cysteine
Allyl cysteine
Benzyl cysteine
2-Hydroxyethyl cysteine
Methyl cysteine
Ethyl cysteine
Propyl cysteine
1-Propenyl cysteine
2-Carboxypropyl
cysteine
2-Carboxypropyl
cysteine
2-Carboxypropyl
cysteine
Propanethiol
2-Propenethiol
Ethanethiol
Methyl cysteine sulfoxide
Ethyl cysteine sulfoxide
Propyl cysteine sulfoxide
1-Propenyl cysteine sulfoxide
1-Propenyl cysteine sulfoxide
Methane thiol
Ethane thiol
Propyl thiol
Allyl thiol
Allyl thiol
Methyl cysteine
Ethyl cysteine
Propyl cysteine
Allyl cysteine
Allyl cysteine and allyl
cysteine sulfoxide
Propyl cysteine and propyl
cysteine sulfoxide
Propyl cysteine sulfoxide
Allyl cysteine and allyl
cysteine sulfoxide
1-Propenyl cysteine sulfoxide
Propyl thiol
Propyl cysteine
Allyl thiol
Propenyl cysteine
1-Propenyl cysteine sulfoxide
1-Propenyl cysteine sulfoxide
Propyl cysteine sulfoxide
Allyl cysteine sulfoxide
Ethyl cysteine sulfoxide
Reference
Granroth, 1970
Parry and Sood
1989
Turnbull et al.,
1980
Prince et al., 1997
Granroth, 1970
Ohsumi et al.,
1993
Hughes et al.,
2004
Figures
A
B
O-acetyl serine
glutathione
alk(en)yl donor
S-alk(en)yl donor
S- alk(en)yl cysteine
S- alk(en)yl glutathione
glycine
γ-glutamyl S- alk(en)yl cysteine
γ-glutamyl S- alk(en)yl cysteine sulphoxide
glutamic acid
S- alk(en)yl cysteine sulphoxide
S- alk(en)yl cysteine sulphoxide
Fig. 1. The biosynthetic pathways proposed for Allium flavour precursors. Route A, after
Lancaster and Shaw (1989) involves participation of glutathione and γ-glutamyl
derivatives. Route B, after Granroth (1970) is consistent with a role for serine
(adapted from Jones et al., 2004).
serine acetyltransferase
serine +
acetylCoA
O-acetylserine + CoA
cysteine synthase
O-acetylserine + sulfide
cysteine + acetate
Fig. 2. The biosynthesis of cysteine (adapted from Leustek et al., 2000)